My Albizia lebbeck / Flea Tree is growing quickly (see photo) and this is the result after only 3 months. I would actually prefer to keep it this size and "fear" how it will look 3 month from now if I don't do anything... Should I prune it? And how?
Any advice is helpful to me.

Hello there,

I would pretty much advise you to let it grow for now. You have a very young tree there that needs to develop in a healthy way. If you prune it now you will do damage to it as the tree feeds through the leafs by doing photosynthesis. I would personally say it is better to let the tree develop and then have lots of options for pruning and styling than doing it now and hurt it.

All the best!

I second Rong on that one. Also put your tree outside if you can, it will grow faster. Make sure you put it in a larger pot if need be so that you can get even more growth. It looks healthy, so that's a good start.