Joining 3 5s2p tool bats to make 15s2p, can i charge them together with an inexpensive charger?

I just purchased (3) 20v 5ah (5s2p) power tool batteries. I would like to connect them in a series to make (1) 15s2p 56v 5ah battery. Connecting them is fairly straightforward, however I would prefer to charge them all together while they are connected if possible. I would also prefer an inexpensive charger if possible, and if I could undercharge them by a volt or two 15s = 63v, I would prefer 62 or 61v charge. I don't care how fast they charge, even 1/2 an amp would be fine. I have a few ideas but I am not sure if this is encouraged.

One idea is to charge them all together a certain number of times, then charge them separately on occassion to balance them.

Another is perhaps there is there some sort of circuit board that the 3 batteries could be attached to that would balance them. I will need to take them apart occasionally for travel, thus the reason why I chose to join 3 sub 100wh batteries (as well as cost).

I guess another option might be a lipo balance charger made to charge 3 batteries, I assume I would have to disconnect them from each other to charge them this way though.

Lastly I can always disconnect them and charge them individually with the power tool battery charger.

Idk, this is all I can think of at the moment. Any suggestions? Inexpensive and simple is what I am looking for

I think what I am looking for is if there is some sort of bms that will treat each of the 3 battery packs like they were 3 individual 21v cells. This way it could balance their charging, etc between the 3 so I could charge them together in a series. Does something like this exist?