Hi @Alex Corral :
Great discussion above! I would add that the closing costs should not be forgotten. That money can essentially be considered prepaid interest (it's money you pay that goes to someone else). These costs increase the effective rate that you would be paying on the mortgage should you choose to go that route.
Going back to your original post, the key to your strategy appears to be the focusing all available cash flows on repayment of the HELOC. This can be effective if you are able to find a good HELOC promo rate and repay your balance within the fixed-rate period. You estimated 6 years to repay your first 3 properties which seems reasonable for a promo fixed-rate period. This will mitigate the risk of rising rates while still providing you liquidity on amounts repaid and you save on the closing costs. win-win-win.
Once your promotional period is up it will be up to you (as it is for each investor individually) if the liquidity of a variable-rate HELOC is worth the interest rate risk going forward. If all goes to plan, however, you will also have equity in 3 free and clear properties...