P2SH- addresses that start with 3 are nested segwit
This is not quite accurate. Address which start with 3 are pay to script hash addresses. The script may be any number of things, from multisig to nested segwit, or 1000 other things. Therefore, although all nested segwit addresses start with 3, not all addresses which start with 3 are nested segwit. It is impossible to tell what the script of an address is until it makes a transaction and reveals said script (unless of course you own the address and set up the script yourself).
Bech32 is valid for those services/clients who have upgraded to recognise it.
I would add to this that if you are using a service which does not recognize native segwit addresses, then you move to a new service. If they have been unable to implement such a major change in the 3 and a half years since it went live, then they are almost certainly also far behind in terms of security updates, patches, vulnerabilities, and so on, not just with bitcoin but with their entire back end.