If you want to do some flips or BRRRRs then you could consider taking out a Line of Credit against the three properties so you can use the cash with interest only payments for a short term project and then pay it back and then use it again (Lines of Credit are revolving - meaning you get to keep using the money).

Since living here I’ve put in a new HVAC in both units and kitchen into one unit, I’ve max out my credit card, and have had my parents help me.

I'm not sure what your local options might look like with a small community bank or a credit union. 

Working with local credit union to refinance at lower than market rates

STRs give you more tax benefits upfront, but it’s wise to choose a property that could perform as an LTR just in case for stability.Using HELOC for Down Payment:The interest on a HELOC used for investment can potentially be tax-deductible, so using it to supplement your down payment could work in your favor.

What documents do they require, what credit scores do they allow, how do they verify previous rental history, etc.?

If you’re running a MTR more like a STR, then you can decide to charge the full boat as agreed or, if you desire (possibly to obtain a good review) find a substitute renter for the period in question and credit the original renter.If, like me you operate a MTR more like a LTR just for a shorter period of time, you probably use a specific rental form provided by either the state real estate commission of the local NAR affiliate.  

(Not on the local MLS) While in Nashville at a doctor appointment, I guess the algorithms were in my favor and it appeared on my phone.

I do not use the lease agreements provided, I use the promulgated contract provided by my state, but it's perfect for background/credit checks, docusign, and payments. 

You can get up to 75% of the ARV on a cash-out, with rates starting at 5.875% APR and a 680 minimum credit score.