The latest draft has finalized "h3" as the ALPN identifier.

https://datatracker.ietf.org/doc/html/draft-ietf-quic-http-34#appendix-B.1

• Removed draft version guidance; added final version string

https://datatracker.ietf.org/doc/html/draft-ietf-quic-http-34#section-3

A client MAY attempt access to a resource with an "https" URI by
resolving the host identifier to an IP address, establishing a QUIC
connection to that address on the indicated port (including
validation of the server certificate as described above), and sending
an HTTP/3 request message targeting the URI to the server over that
secured connection. Unless some other mechanism is used to select
HTTP/3, the token "h3" is used in the Application Layer Protocol
Negotiation (ALPN; see [RFC7301]) extension during the TLS handshake.

https://datatracker.ietf.org/doc/html/draft-ietf-quic-http-34#section-11.1

This document creates a new registration for the identification of
HTTP/3 in the "Application Layer Protocol Negotiation (ALPN) Protocol
IDs" registry established in [RFC7301].

The "h3" string identifies HTTP/3:

Protocol: HTTP/3

Identification Sequence: 0x68 0x33 ("h3")

Specification: This document