Only income from your side businesses can be used to fund for your own solo(k) [assuming you have no employees].Additionally, if you have a 401k from your employer, in the employee bucket you are limited to $18K for 2016 across all your 401k's and the employer bucket for your side business is approx 20% of gross income.

My first goal to to make 3k a month, then the sky's the limit!

Check your state landlord laws for the amount you can charge for pet deposit as some states have limits.

Considering the idea of house hacking but don't necessarily want to limit our scope to Greenville.

In most cases, it does not, because the key benefit of the Solo 401k is the higher contribution limits such a plan provides - which you would not likely be taking advantage of.More likely, a self directed IRA LLC would be a better option for investing retirement savings into real estate.  

Generally speaking, it limits your personal liability, is easy to manage, and has pass through treatment for taxes.

Now that you have exhausted your Low Down Payment Options (Portfolio-Conventional and FHA) you will need to use a Standard Conventional Loan and put 20% down to get your 3rd Duplex.The sky is the limit...you could continue to buy properties and build an empire or just stop there and enjoy more free time...I have friend who quit his job and just manages his properties now.Be alert, if you time it right you might be able to get down payment assistance/grants (I got $5,000 from a local program and used it to buy my interest rate down to 3.125% which is way below typical market rates for a Duplex).

Reg A has been around as along as Reg D,  but had a $5M cap and just wasn't worth the attorney expense and regulation limitations.  

We like to sell properties as a "Rent-To-Own" which also reduces the amount of turnover and repairs as well as limit the amount of management headaches on a property. 

While the going is good, the sky will be the limit on his and most others' labor rates.Best wishes with this.