well if you are talking about a cashmini ISA then then it;s like any other savings account but wit no tax. These days most of them calculate daily and update monthly or annually but there are all sorts of variation.
Yes, as R1G says, if you are getting 3%, that means for every day the account is there, one gets (the capital)*0.03/365 in DAILY interest. This is added to the capital sum typically once per year (such that one then starts to accrue interest on the added interest), though some accounts may add interest the accrued interest more frequently.