Children do things like this for example, as a way of "controlling" their environment. They may be capable or not...but the way they "communicate" is also through actions such as this. It also creates a safety-zone for them... their comfort zone. It's a natural developmental behavior...although sometimes it may not be convenient or acceptable. Regardless, it is a child's way to bring order and control to their world, especially when they are stressed. Children also, can experience "stress" even though we may not think of it.
You seem to have tried many things, that most Moms do. You seem very patient as well. Good for you! 3 years old is also, still, a very fickle age....their emotions are developing and their sense of independence as well as "dependence"....they are "big girls" but are still in a sense "babyish" in maturity...3 years old is pretty much at the cusp of a maturity leap and lots of developmental changes. It's not easy for them either, because at this age, Parental and adult expectations of them, at this age, increases. And afterall, it's "Preschool" age. Another big thing and adjustment.
Your girl has many things to adjust too right now.... potty training, entering into Preschool, and your pregnancy.... and the "idea" and concept of her having a new sibling. Having a new sibling is not tangible to them yet, until the baby is actually here... so until then, they like anyone, will imagine and have adjustment issues.
That being said... perhaps talk with her about it. I know you did that already. Also, try and make her "help" you clean up her pee if she does it on purpose on the floor. They are at the age when they can do simple "chores" around the house... in Preschool they will be doing this as well....in Preschool each child is given a simple responsibility and they call it a "job." Because they are at the age when they can participate like this, and it is good for them to learn responsibility. Then, when her clothes is all wet with pee, have HER put it in the laundry basket etc. Don't "lecture" her about it...but tell her simply that she needs to do it and that it "helps" Mommy. For my girl, she really liked when she could "help" Mommy....it made her feel special and involved.
She may also be balking at potty training because of your pregnancy....it's a lot for a child to handle all at one time. Each child is different in their "coping" skills. Maybe she's doing it because she wants more attention...nothing wrong with that if that is what she needs....maybe this is her way or reaching out for you. Make her a part of you.... and a part of your pregnancy. Maybe she needs to feel closer to you, and "safe" in that she won't be left out etc.
When I was pregnant with my 2nd baby, my girl was just about to enter Preschool too, and she had FINALLY gotten the hang of complete potty training. Just in time. There were a few times she did have accidents....BUT it was usually at times in which she felt stressed or there was a change in her life and in the process of adapting to her "new" life. (as a Preschooler and me being pregnant). I simply explained to the Teacher...and they understood, as they know full well that this can tweak a child's sense of security and well being. My Hubby & I made sure not to "add" to her stress as a child... we never scoleded her or made her feel stupid for having accidents...we talked with her about it, gave her extra attention and love, and spent more time with her... we took pictures of her with my pregnant stomach and made her more a part of everything. We talked to her about her baby brother and how much he will love her too etc. Sure, they are also quite opinionated and stubborn at this age...but they are just children and quite tender hearted...they need a balance... and their acting out like this usually means they are needing something they are not getting.
Well, sorry for rambling on, just sharing my own experience with this that I had with my daughter. Hope it helps and all the best to you,
~Susan