If you are selling a triplex, 2 of the 3 units are investment property so they would not be protected from capital gain taxes.

QOF can be a great way to defer your current tax burden and exclude gains from the new property purchase.The big con is this is a very illiquid asset with a long-hold period.

To reduce capital gains?

This was exasperated when the socialist Labor Party took power in the 1950s, and raised capital gains to where the tax on ordinary income was, 89.9%.  

Try to do a 1031 Exchange to save capital gains taxes.

Cash flow in current unit will go down, and any equity you hope to tap into will be less than if you sell.Don't fall in love with any property, or the idea of paying off a property, until you have reached the financial goals you hope to gain from owning the rentals

Even if it's just a duplex or fourplex, the knowledge and confidence you'll gain from that first transaction will make the bigger deals much easier.

As a long-time LTR landlord, I'd rent them out long-term, but only for a year or 2 to take advantage of the tax free gain. it comes down to market timing ,  really. 

Take a capital gains tax hit of $15K (15% of my gains) and wait to re-invest in a rental when the market corrects.

If there is anyone currently involved in the Opportunity Zone Fund process to benefit from incentive programs to defer capital gains tax please reach out to me.