There is nothing wrong with wanting to get married. But your triad partners are not available for marriage, so you will need to find someone else for that. The beauty of polyamory is, you can. You can date single people who are available for marriage even if your triad partners don't want to date others themselves.
Anyone new you date does not need to be a part of your triad too, although it is possible that your triad partners may want to meet them and attempt to form a friendly connection with them.
There are a fair number of single poly people on OKCupid, or you could try going to poly meetups and events.