Mortgage almost paid on 3 unit rental - what next?

[Allison B.]

Hi, 

My wife is Allison, I'm her husband David.  We have a 3 rental unit in Park Slope, Brooklyn.  The 15 yr mortgage will be paid off in 3 years.  Right now we have 90% equity in the house.  We'd like to make another real estate investment now but not sure what to do.  We rent our current living space in Manhattan and don't have a lot of liquid equity on hand.  I figure my options are: 

A) do nothing and have mortgage free cash flow in 3 years, then save big time for our kids college (about 7 years from now).  

B) We could refi the house, buy another property and rent it that out.   Taking out a 15 yr. mortgage on both aiming be paid off when we're about to retire (at 65ish)

C) leave the Brooklyn property be and get another mortgage on a house that we move into.  Hack that house or quickly flip it.  Then try to buy another house either to live in or flip

D) Wait 3 years, move into the Brooklyn house and live for mortgage free and then make another investment

Anyone have any other ideas?  Part of me is really looking forward to not having a mortgage to pay a mortgage.

Thanks, 

David