Disciplining kids who won’t listen??

I have 3 children; 7, 4(5 in 4 months) and 3(4 in 3 months) and I’m currently 30 weeks pregnant with my last child. I do not have my 7 year old right now (she’s with her dad during the summers) I am having the hardest time getting through to my 4 & 3 year old. They don’t listen when I gently ask them to clean up, my 3 year old goes as far as telling me “no” and/or runs away slamming doors, etc. Most of the time, I will help them when they clean up, but lately, it’s gotten to the point to where they’ll dump all their toys out, and then refuse to clean up on their own, even when I’ve helped them, and now it’s gotten to the point to where they’re doing it EVERYDAY now, so I quit helping. The last 2 days, I’ve gotten so mad at them for not cleaning up, that I’ve taken tv time away, and have even gone as far as getting a trash bag and “throwing their toys away” to try and get them fo clean up, and that does nothing! I am losing my patience. We live with my MIL, and she constantly spoils them, bringing home new toys/treats EVERYDAY, and undermining me when I tell her not to give them something or that tv time is a no right now. I have no clue what I am to do to get through to my kids, if it’s jealousy of a new baby soon, or if it’s that their just that spoiled with no respect for me?? I feel like I’m failing because I can’t ever get through to them in the way I want to, so I resort to yelling and losing my temper.