Maybe have your husband accompany you and the kids to the next dental appointment and have the dentist explain the importance of brushing, flossing, and rinsing.
Hard to imagine a parent would think that good dental care is not important. Wow.
Other than that, keep at it with the kids. Use a daily behavior chart (stars, smilies, whatever) to get them to do the behaviors you want (brushing for two minutes at least twice a day; flossing, etc). They "earn" stars for good brushing. No proper brushing, no star. Once they earn a predetermined number of stars, they can trade those for something they like----a little extra tv/game time; extra park or play time with friends; family game night; extra story at bedtime; a smoothie----whatever, keep it small, simple but something they like and will work toward. Try to stay away from material goods as rewards.
Maybe let them pick out new toothbrushes or get the battery operated kind. Those help with getting up along the gum lines where plaque accumulates. Get your brush and brush alongside them (you can use a dry brush to demonstrate, or if you're done eating/snacking for the night, you can brush with them) to model what you want them to do.
Our youngest two boys are like yours (although one of these is 19! and I just got the bill from the dentist today---yikes)! I'm determined our 8 year-old will develop better brushing habits. With the 8 year-old, if he doesn't do it right the first time, he has to go back and do it again. If it's still not done correctly and I can still see food and plaque, then I do it. He does not like when I brush his teeth, so he usually gets it right so I don't have to! Plus, if he doesn't do it right, he loses his brushing star, and the time it takes to re-do the brushing takes away from bedtime reading. Those things have helped him become more responsible for improving his brushing and dental care.
I know you said your kids are resistant to you brushing their teeth, but with the three year-old, you are most likely going to have to help him and step in. If the six year-old doesn't get it right on his own, you'll have to step in there, too. Be more insistent than they are resistant. Not brushing well is not an option. Make sure they understand you mean this and follow through. It's that important---not just for the teeth, but for overall health.
Good luck.
J. F.