As mentioned in a previous reply in another thread as I erroneously proclaimed to have found the brass Tee fitting coolant connectors at my local "Home Cheapo".
I am here to confess in all humility and embarrassment - I may have not chosen the appropriate connectors as I had initially thought. They are indeed PEX.
I don't know...I assumed a properly tightened worm clamp would work just fine. I don't know.
However...


Albeit while the connector on the left is obviously not the correct 3/4", 3/8", 3/4" tee (in lieu of the 3/4" to 5/16" Reservoir tee), I intended or thought I saw on display; Not only is it not 3/8" (but 1/2") - the orientation of the reducer is wrong.
I inadvertently grabbed one off the rack in haste without paying attention to avoid being late for work.

Furthermore - you were right. I went back to Home Depot and while they do have 3/8" tee connectors; they don't have 3/4", 3/8", 3/4" reducer tee connectors like I thought.
My bad.

Mcmaster Carr was also a dead end. I went with both your eBay finds: Brass 3/4"/ 3/8" tee and Aluminum 3/4" / 5/16" tee.

 

Thanks for this. Not to worry, I sourced two through Amazon and the third through an online plumbing supply. I didn't have time to wait for the overseas shipment for the aluminum one. These three should all arrive tomorrow. Planning to post my experiences with them in a separate thread if they're good; feel free to chime in / pile on the discussion if you like; the more information the better. The de-plasticizing struggle goes on, never ends with these cars...

 

Although, I'm not in the process of working the cooling system right now, I'm just in the looking forward/preparedness mode of collecting back-up parts.
I also found some applicable 3/4" BRASS connectors on Amazon from - @ a lower cost and better yet ships from Amazon, if anyone is interested.
We are lucky enough to have several major Amazon distribution centers here in my metro. So I've never had a need to subscribe to Prime. I usually receive purchases within 1-3 biz days.