My background includes real estate sales,  mortgage lending and business lending.

(ie if the total arv for 3 unit is 1.3M, and he spends 450k on hard construction cost, 250k in land and permit, plus lending + closing & etc costs 100k for a total of 800k, his equity will be 500k out of 1.3M or 38% - just assume everything is correct)Thank you all.

But as I began growing interested in real estate as a form of passive income I was advised that credit is akin to a résumé for financial institutions.

Check with a bank and what their lending criteria is.Heloc's in my opinion are awesome! 

I say this because I am a private lender and we lend to people trying to do just what you are doing.  

Since you are looking for a blanket loan for multiple properties, there won't be a fannie mae restriction and it will be up to your local bank (or whatever lender you use) to determine timing on when they will lend, and how much they'll lend.

I actually have seen where portfolio lenders advertise using one property as down payment on another, but have not experienced this, and when I have spoken to their loan officers, generally do not get an answer on whether they would actually do it.More importantly, you will learn that even with a cash-out refi, you will likely not find anyone who will lend more than 70 - 75% of appraised value, and with the cost of the loan, it would likely not be worth it to you for the 10 - 15K you could potentially take out if you currently owe 60K on the 1st mortgage.  

I had always thought that it was specifically about how banks and people lend out funds and that's it.Anybody out there that can shed light on this?

A funny thing happens in commercial lending.

I can understand if you don't know the buyer and their lending situation, it is hard for you to vouch for their ability to close.