If I have 3 separate networks, can I use 1 NTM license?

I dont understand the query completely, say suppose you have Network A, Network B and Network C and you have placed NTM on Network B IP Address. If NTM that is installed on Network B can talk to Network A and Network C then 1 license is sufficient.

But if you are planning to install NTM in all 3 Networks (which is NTM in Network A, NTM in Network B and NTM in Network C) then you will need to purchase 3 separate licenses to the best of my knowledge.

YES. 

NTM License is a stand alone. You can install NTM in your laptop or any stand alone machine. From that laptop you can scan any network you like. 

Eg:

Connect your laptop to scan office a network. Once done, connect your laptop to office b network and scan it. Do the same to other and so on... 

https://support.solarwinds.com/SuccessCenter/s/article/Guide-to-SolarWinds-Product-Licenses?language=en_US#NTM

https://documentation.solarwinds.com/en/success_center/ntm/content/ntm_getting_started_guide.htm