It depends on the rules of each Uni, but in most cases there is nothing stopping you.
You are not financially liable for course fees if you pull out by 30 March, but you might not get a refund on things like SSAF fee.
Many students want to do things like this to give some more time to decide. However
1: if you haven't decided now, is a few extra weeks going to help you?
2: if you want to "taste" each Uni first, you are putting yourself at risk at falling behind very quickly, because you are trying to taste three courses at once with lectures and assignments x 3, will miss stuff if you are at the other uni, make it harder to find friends etc etc.