Next will be, what must I spend to supplement that work and create 3/month?

You're thinking to buy a rental property and use the rental income to supplement the travel kitty. 

I am new to this, and I am hoping to supplement my current income so I don’t have to work so hard in the hospitals.

These would be a supplement to your BHA and also provide cash flow once you move on to the next duty station. 

The only reasonable explanation I can find is they are supplementing with J.O.B. income, or perhaps living off the excess borrowed funds..Some part of their story is missing.

I'm hoping to earn some extra income to supplement my day job.

If that's not an option, getting a side hustle to supplement income could also work.

Other houses that I have that are strongly cash flowing I can use to supplement my income and help with operations costs when starting into the MF property.I have also invested in a pair of fourplexes, which is not quite the multi-family model (which is 5+ units) but a good starting point with a larger supply of units so a vacancy does not hurt quite as much.

So the question becomes, are you more interested in cash flow to supplement you income or are you more interested in real estate appreciation for a larger sale value?

I’m happy with supplementing monthly expense down to triple digits, or realllly low 4-digits.