I'm the stepmom and I take my SD out to buy gifts for her mom for her mom's birthday, Mother's Day and Christmas. Her mom does not do the same for us, but that's okay. I do it for my SD, so that she can feel a part of gift giving.
The 19 year old needs to figure out how to buy gifts. Give him money and a list of suggestions that you work out together.
You'll have to take the 3.5 year old out, but remember--the gift should be from the heart so let your 3.5 year old pick out something you think is appropriate from him/her. Then it won't feel so much like you're giving a gift from you with your 3.5 year olds name on it!
Lucky for me my husband's ex was always very nice to me, but the turning point was when my SD showed up on Mother's Day with a gift for her mom that I helped her pick out. Evidently my husband never did that so she knew it was my influence. Now we have a great relationship and it works out the best for my SD!
Merry Christmas!