These are sample lines to add to sslproxy.conf, assuming you have built/setup the test env with H3 support:
# 5 ICAP services in chain
Icap icap://127.0.0.1:1344,squidclamav,squidclamav,yes,yes,10,1024
Icap icap://127.0.0.1:1344,ex206,ex206,yes,yes,10,1024
Icap icap://127.0.0.1:1344,echo,echo,yes,yes,10,256
Icap icap://127.0.0.1:1345,reqmod,respmod,yes,yes,10,0,0,no,no,X-ICAP-E2G
Icap icap://127.0.0.1:1344,suricata,suricata,yes,yes,10,1024,0,yes,no,X-Response-Vars
# H3/QUIC at 8444, Caddy at 8443 (UDP)
ProxySpec http3 127.0.0.1 8444 127.0.0.1 8443
# Point H1/H2 clients to upgrade to H3 proxy above
RewriteAltSvcPort 8444
# H1 and H2 at the same H3 port numbers (TCP)
ProxySpec https 127.0.0.1 8444 127.0.0.1 8443