Congratulations on getting to 3 months.
Giving the odd bottle of forumla (one or 2 per day) is fine - I know with DD, she went into creche at 3 months (less mat leave then) and I didn't always manage to pump enough so on average, she'd get 1 or 2 BM bottles but 1 or 2 FF bottles per day at that stage.
The rest may give you a chance to increase your milk production - allow your LO to take as much as they can at each BF. And maybe finish off those feeds by trying to pump any excess for later (and even have that for your DH or someone to give at a different time, like a late evening so you can have an early night to rest).
And even if you want to switch entirely, take it slowly or else you will have problems with milk still coming. So drop one feed every couple of days.
I managed, with the use of formula, to do mostly BF until 8 months (all night time and early mornings, and pumped until 7 months for 1 bottle/day in creche; dropped back to 1 late night and 1st feed of the day by the time we were going on hols at 8 months which was handy for the plane), and we finally dropped the last late night BF feed (in fact, dropped it altogether) when she was just over 9 months.