You could also create just a single member LLC and hold your share of the properties in the LLC so that you could combine both your 50% share of the jointly owned ones plus your solo-owned properties, and then you would need to change title to the jointly-owned ones to be owned 50% by your LLC and 50% by your partner however he decides to hold it.You probably should have already been filing taxes in South Carolina if you haven't been already as a nonresident on the income earned from your South Carolina based real property (individual income taxes). 

Drive to stick with it for the long termI think this combination of skills is important.

How to buy this land with combined money from all 3 of us?

Combining both hard money and a HELOC could work.

. ~$410/month in HELOC interest paymentsInitial Cash Flow/COC return: $599/mo cash flow, 12.95% COC return*Note: Cash flow assumes 15% expenses combined for vacancy/maintenance/capex, plus $150/mo water/garbage bill, and $75/mo for lawn/snow.

A year and a half later in 2020, I took the two short term rental income I received from those rentals, combined it together with W2 once again to purchase my third one.

From my elementary knowledge of tax law I found that the IRS can determine a MLO to be an installment sale if the terms look like an installment sale, aka there is a large down payment and combination of the lease and option equal the fair market value.

My end goal is to produce $40k-$50k income per year from all of my real estate ventures combined. 

3) Your combined up-front + ongoing repairs budget seems low (3.5% up-front + 5% ongoing). 

I use a combination of BP calcs (not the most user-friendly if you want to make changes or run "what-ifs"), DealCheck.io (I like the UI a little better), and spreadsheets (customization for unique cases).