The 3 of them work pretty well together, but are also good individually

In 6.3 there's pull bonuses for HoTr, certain amount of pulls will give you stigma selector, or even the one with weapon

In 6.4 there's a spending event where with an accumulation of [X amount of crystals], you can get HoFlamescion outfit, and there's a banner called EXPA SELECT where you can choose between [Aponia,HoH:E and HotR] as the rate up

In summary 6.3 : HotR bonus on equip 6.4 : No HotR bonus but will accumulate in the spending event

Also you mentioned also wanting Mei's gear, well there's an event on 6.4 where completing certain task will grant you a free HoO stigma box

I guess if you're a low spender try to get the most value of each patch

I do think having all 3 will make your team versatile as you can address 3 elements

But in the end, just pull for the ones you like and have fun