Is this your child, or are you a sitter, or a nursery school teacher, or what?
I would say that the pelvis thrust is more concerning than WHAT he is saying. you need to carefully evaluate who he's around and what they're doing in front of him...and make sure he's not seeing this on tv. it may be time to have a talk with parents of the other kids that he hangs around and see if anyone else has noticed this cropping up. if none of the parents of the kids he plays with have noticed something, you need to look in your family.
Well, he's definitely heard something, but it sounds like he's just copying it blindly rather than indicating he has had this really happen to him. Who does he play with, older boys?
he makes comments such as eat my peepee and thrust his pelvis at me
It can be normal but it depends what the child said.
Could you tell us what she said its hard to decipher otherwise ...
It depends on what you mean by sexual comments. If the child has heard an older kid using a potty-mouthed insult, such as 'you m-th-rf-ker, go f- yourself," that is not in fact a sexual remark but trash-talking. A smaller kid might be impressed by the grandiose sound of the remark (without understanding its ramifications) and try it out on someone else, and the person he is likely to try it out on is one of his buddies. Heck, adults use that kind of insult, and they are not suggesting the person really go do it.
If the child is saying "Let me put my *** in your *** and pump hard," then that is a sexual remark, and I would be concerned about the specificity of the comment and worry he or she is being abused.
If a kid is asking his friend something about "does your pee pee stick up sometimes?" that is simply more likely to be curiosity about the body.
No. That ain't normal. The child is 3!! He/She has prolly heard some adult making some comments, or seen some movies he should not have. Their minds are like sponges, always absorbing new words and things around them, and it sounds like you a a pretty smart 3rd old. I'd suggest you buckle up and address this situation.