Triathlons could still be your thing, if you get some of those flexible knee supports. It then becomes the thing of trying a run for certain length of time, to see what you feel and then assess that, with maybe an exercise bike session and a swim, to see if you are capable of getting a good basic performance level and mindset with all 3, that could be improved on and then consider taking it from there.
You could also consider certain supplemental options for the Knee like Chondroitin, or Hyaluronic Acid as examples. Chitosan and Glucosamine are no good if you have a Shellfish allergy, owing to the Chitin and Glucosamine has been proven more recently to be not that good for Osrteoarthritis, but basic Knee jip I'm not sure about, but I'd say it's probably not fantastic for that either.
If you are experiencing any grating noises in the Knee, obviously I'm not sure if you are or not, it might be Patellofemoral Syndrome, A.K.A. Chondormalacia Patellae, A.K.A. Runners Knee, Housemaids Knee and Secretaries Knee and it is caused by degredation of the Cartilage, allowing the Kneecap to slide to the lateral, (outer), edge of the Knee. Chondro relates to Cartilage, Malacia means softening, both words derived from greek.
This could be overcome with strengthing exercises to the Knee and something like Hyaluronic Acid might works as H A is a component of Skin, Eyes and Cartilage, or a product like Cartazyne DS might work as well as I believe that is stil around and is made to directly target the Patellar area.
The main thing of course though, is to find what works for you and stick to it. Crossfit isn't bad, but has a reputation for heightening injuries, due to the often below standard implementation of techniques, so you must consider this as a minor caveat when doing Crossfit, because unlike normal weight training where you could see what to do and get it right most of the tiem, seeing a crossfit move performed properly isn't necessarily going to guarantee, you'll do it right within the first 2-3 attempts, it could take several sessions to master it and be consistent at it, but is has worked for many is widely regarded as a pretty credible exercise option.