Hi emailsms.
Welcome to our Mattress Forum. 
Sorry to hear about the pains you’re having on the mattress, as if you don’t have enough of that to deal with during the waking hours!
Do you still have this bed? Memory foam tends to collapse under body heat, doing what you’re explaining here - becomes very (sometimes too) soft and removes the support we’re looking for (depending on who you are and what you need, of course).
The good news (if you can call it that) is that it’s relatively “easy” to soften a too-firm bed, but there’s not much one can do for a too soft bed.
Are you a back, side, stomach, or mixed-position sleeper? 3" may be more than you need depending on your sleeping profile.
I’m fairly certain there are some non-profits that help people get mattresses, but I’m not too sure about other bedding components. I wonder if you were to go into a mattress store and inquire about their floor models (like the toppers people test) if they’d sell those for a discount? Maybe our experts here in the forum will have a few ideas on that front.
This list is old so it’s possible these stores are no longer open, but I figured I’d drop them here in the event they’re useful. Not sure how far you are from Indianapolis! Here’s an old list for Louisville as well.
NikkiTMU