Condo multi family and sell one unit?

[Meryl C]

I have a nice and well-maintained three-family in a desirable area. I'd like to condo it, but not sure I want to sell all the units right now. Ideally,  I'd like to sell one, which would enable me to pay off the mortgage, and continue receiving income from the other two.
is this legal? Will it make it hard to sell the other units? I seem to remember from a prior condo that Having one owner own more than one unit might be a potential lending issue?

The other downside I could think of is that the taxes will go up on all three units once I divide it. But is there anything else I'm not thinking of that could be a drawback? And are there also some scenarios where this would be a good idea?
The mortgage is over 300K, so I would love to get rid of it.

Thanks for any advice.