Hey there @i_yu
You certainly can.
The easiest way should be to text PAC to 65075 from your Three pay as you go phone.
You should get a message from Three with your PAC (port authorisation code - the code that allows you to port your number to a different network).
You then give this code to EE - all the details you need can be found on this thread.
Once you've done this then your number with transfer across to EE the next working day (if you've completed before 17:00). Your Three sim and account will close.
Happy porting 😀
