Probably you have some wrong items in your Transunion credit report.
You should use credit repair agency to fix your credit – for example this one – - They can clean lots of bad stuff from your credit report – and do it much faster than yourself, so your credit will go up fast.

Alot of people use

Most major credit card companies use all three credit bureaus.

If your TransUnion score is considerably lower, you may want to pull a copy of your report to see what is showing. You could have an error that needs to be disputed.

No way to tell. Most use more than one bureau.