I can completely understand your dilema, especially with a 3-year-old and living with the uncooperative in-laws. I have been there just a few years ago when I first returned to work as my daughter started kindergarten, except I moved back in with my dad after separation from my ex-husband. This is what I recommend:
1. It is imperative that you spare 45 minutes to 1 hour for a daily "quiet time" alone so that you can sort out your daily short-term goals and make a to-do list. You might need to get up a bit earlier everyday to do this, but it helps tremendously in keeping your focus. Once a week, hold a slightly longer quiet time to review and evaluate your long-term and short-term goals, as well as sit down with your husband to go over them. This personal space is crucial for your health and sanity's sake, as it helps to relieve your stress. You can even listen to some relaxing music while you plan. Also remember to schedule some relaxation time for yourself at least once a week, such as take a bubble bath or take a nap to relaxing music or draw or do whatever you used to enjoy doing but no longer have the time for. I used to feel guilty about spending time on myself, too, but you MUST think about the big picture for your kid's sake. Everything you say and do serves as an example for your child, so you must do this to stay focused. Carry this planner everywhere you go as a way of showing how serious you are about it.
Start your first session by writing down your long-term and short-term goals, such as "I want to be a better mom in these ways...." or "I want to get these things done today, in 1 week, 1 month, 3 months, 6 months, or 1 year...." Then make sure your daily tasks and routines contribute to those goals. For example, "We need to be out the door by 7:30," get a timer and set it on the table; ask your husband to supervise your child's breakfast while you get dressed for work. Then switch places: you supervise and help your child brush teeth, potty and get dressed while your husband get dressed. Since you have other adults in the house, try to delegate some tasks.
I recommend getting Franklin Covey's planning system. You don't need to get their expensive binders, they have cheaper ones available at Target. Or you can just get their "Start Up" packet and incorporate that into your planner:
2. After your first quiet time session, sit down to have a serious talk with your husband. Ask him to be on the same side as you with the parenting. Tell him his belittling comments hurts your feelings and eventually deteriorates your marital relationship. Tell him how serious this matter is to you, him and the kid. This is absolutely necessary for the well-being of your child, so talk to him from this approach. If you want your child to be in bed by 8:30, he cannot say it's OK for the kid to start before-bedtime routines at 9:00. If you say no candy after 5 p.m. (to prevent hyperactivity before bed), he cannot let his parents give the kid candies after that time. Ask him what he wishes for the child to become. Engage him in this parenting planning process. If your husband is still stubborn about belittling your plans, let him know the serious consequences of raising a confused child. Remember, you are the parents, not the in-laws.
3. As for the in-laws, I'm sure they have good intentions, too. So the next time they attempt to trump your rules, confront them directly in a polite way. Say something like, "I really appreciate your experience in parenting/concern/perspective as grandparents, but I must enforce this rule because....." or "I am very grateful for you to allow us to stay with you during this time of hardship, but as the mother, I must enforce these rules...." and tell them your reasoning. Finalize your statements with "Please do respect that." If they try to start any argument or "debate," turn to your child directly and make your statement, "No, you may not have any candy right now. You may have some tomorrow, but right now is too late for that. Go ahead and play with....." Or you can offer him/her some other kinds of snacks, but take the candies away. If it's bedtime and your in-laws tries to say it's OK to stay up 15 minutes or 30 minutes later, just get the toothbrush out and put the toothpaste on it and hand it to your kid. Then say, "After you brush your teeth, let's go read your favorite bedtime story." If there's any temper tantrums, enforce the time-out rule.
Show your child that you say what you mean and mean what you say by following through with action on all of your words. To show this, even if you're in the middle of grocery shopping and he/she starts a tantrum, be prepared to drop everything and walk out of the store. If the tantrum starts in the car, immediately pull over somewhere or turn the car around to go home, and tell your child to stop it immediately. This types of actions show your child that you are deadly serious with your requests....so that if you say he/she needs to finish the meal and doesn't, then you will not give extra meals and snacks later.....or if you must head out the door by a certain time and he/she is still not done with the breakfast, then you still leave by that time and no food until lunch time. Do this even if you don't have your husband's support. You can always have a talk with your hubby later----in the absence of your child----don't let your child ever see you fight or argue with another adult. You MUST reinforce your words with ACTION----don't just say something and let your hubby or the in-laws override your words. It's kind of hard at the beginning to state your wishes and stand your ground, but after a while, everybody will see how serious you are. Don't do everything all alone, you can engage the in-laws' help with the babysitting, too, but just be firm about your rules. Delegate the house chores, too. Tell everybody your schedule and reasoning behind those rules. Most of all, you must enforce it with action. It took me over 3 months of daily reinforcement to get my daughter to school on time and over 6 months of daily action for my dad and the ex-in-laws to respect my boundaries, but not backing down on my words really worked.