├── multibitbtcfirst.key
Multibit had 2 ways (I know of) to save this file: with encrypted private key(s) or with clear private key(s).
There are commented line starting with #, those we ignore.
The normal lines contain the private key, a space and a date.
If you have multiple lines you may have had more addresses there and you have the private keys for all, separately.
If you import private key into Electrum and it doesn't work, then it's probably encrypted and you'll have to remember the password; in that case you should install Multibit (Classic), import the .key file, then export the key file without password. (
on a safe computer!). Then import the keys into Electrum, see that you have the funds. Then create in Electrum a proper new wallet with seed. And transfer (all) the old coins onto a new address of yours. And keep the seed safe.
I don't advise you send the coins from Multibit directly because it's buggy.
Edit: 3rdinvestmentwallet.key may be Bitcoin (Multibit) or may be Dogecoin (Multidoge); does this ring a bell maybe?