Hey there!
The first step is pre-bonding. Switch them between cages every other day, this gets them used to each others scents before the actual bonding process. Bunnies are very territorial, so territorial that they will fight to the death If a strange bunny looks at them wrong while on their territory.
Getting them used to each others scents makes them less likely to fight or have negative emotions towards each other when you actually start the bonding process. Sometimes it is easier to introduce each other in pairs, and then move onto having a three-way bonding session.
I highly suggest starting in completely neutral territory. Somewhere none of them have been before. This can be a coffee table, in the garage, a room in the house they’ve never been in, etc. This makes them less likely to be territorial towards each other.
Expect lots of pooping, I suggest putting out a nice pile of hay, veggies, and other treats. It helps associate another bunnies scent with a positive activity, all buns like eating lol.
After a week or two of pre-bonding, start with small sessions. 5-10 minutes is suggested at first, after a few sessions, slowly increase by five minutes. So from 5, to 10, to 15, etc.
There’s been a few trio journals on here, I suggest giving them a read to get some tips. It could be helpful! I’ll make sure to link some!
Luna & Sirius’s Journal: https://binkybunny.com/FORUM/tabid/54/aft/165467/Default.aspx
Mikey’s Journal: https://binkybunny.com/FORUM/tabid/54/aft/145198/Default.aspx
LittlePuffyTail’s Journal: https://binkybunny.com/FORUM/tabid/54/aft/111359/Default.aspx