Hello and welcome!
I have three SFH as well and am working on my first duplex deal currently. I just found out a few things as I am going through a HUGE renovation on the third SFH (currently living in it as primary residence). I was unable to do a HELOC on a non-primary residence. I can do a HELOC on the SFH I am currently in, but they take the purchase price as the appraised value for the first year (only helps if you put a decent $ down). After that first year they will allow for a new appraisal and HELOC depending on amount. If you do a cash out refi, most banks only do 75% loan to value ration. For example if your home is worth or appraised for 100K. The bank will only give you 75K. If you owe 70K, this will not be a beneficial transaction as there is still closing costs etc. that need to be accounted for.
My first home was a huge rehab but didn't change layout and that one is doing very well. Second home was also facelift criteria and also rents very well. This current home is in a very high end market and am having to sink waaay more money than I had planned to either flip or rent. Luckily I have at least 200K in equity in it after purchase, but this extensive renovation (knocking down walls with high end finishes) would not have worked with my other two properties.
I hope this helps! :)