Hi and welcome!
The way to cluster feed any baby is just to offer another feed at a shorter interval towards the end of the day. At 3 weeks BW babies are on 3 hourly feeds, which also helps to get more calories in them during the day rather than the night. If a baby wakes at 7 for example feeds would be 7, 10, 1, 4 and then to cluster you would feed at 6 then 8 and a dreamfeed between 10 and 11pm. My two never woke at a regular time so we just ran the day from their wake up time.
I will say though that expecting to drop the 2am feed in a couple of weeks may be a bit unrealistic. In reality, even with clusters (clusters are dropped for around 8 weeks anyway) and a dreamfeed most babies will need a nightfeed until they start solids around 6 months. The clusters and dreamfeed are meant to get you a longer stretch until the nightfeed not to allow them to sleep until morning without a feed when they are so little - in fact, sleeping through actually means a 5 hour stretch from the last feed. Some babies will naturally drop the nightfeed quite young and that's fine but as they have so many growth spurts in their first 6 months (3 weeks, 6 weeks, 12 weeks, 4 months, 6 months) you really need to feed if they wake.
Formula takes longer to digest than breastmilk so it's common for FF babies to refuse cluster feeding, both of mine did so I would work on a 3 hour EASY pattern and try establishing a dreamfeed if he won't take a extra feed. Don't forget the DF is an asleep feed, just pick him up gently and lay him back down after!
Laura