1. Create .pem files for each domain of key, crt, and intermediate cert (if any):

sudo cat DOMAIN.key DOMAIN.crt CRT_intermediate_sha2_sha1root.crt  > dom1.pem

2.  Edit haproxy file and have list of domain.pem files bound to port 443. Note: you also need mode=http, some variants shown

defaults 
   mode http
...
frontend webfarm443
       bind *:443 ssl crt /etc/ssl/dom1/dom1.pem 
                      crt /etc/ssl/dom2/dom2.pem 
                      crt /etc/ssl/dom3/dom3.pem

       # map host names to backends
       #Domain1
       acl host_dom1 hdr(host) -i DOMAIN1
       acl host_dom1w hdr(host) -i www.DOMAIN1
       use_backend DOM1_443 if host_dom1 or host_dom1w
      #Domain2
       acl host_dom2 hdr(host) -i DOMAIN2
       acl host_dom2 hdr(host) -i www.DOMAIN2
       use_backend DOM2_443 if host_dom2
       #Domain3 
       acl host_dom3 hdr_dom(host) -i DOMAIN3
       use_backend DOM3 if host_dom3

3. Test config: 

sudo haproxy -c -f /etc//haproxy.cfg