For the 3 bitcoin address types, which can send to each other? Can a valid tx be created that spends to an unspendable address?
What you can spend, and what you can send to depends on the wallet software and nothing else.

There are no inherent restrictions on any combination. 

In case someone uses old software, they may not be able to send to bech32 addresses. But they'll get an error that the software doesn't recognize the address; no funds will be lost.