Haven't done the Triangle - per se - but have done all parts numerous times.
Part 1 - San Juan Islands to Glacier/Waterton National Parks
Fly into SeaTac - shuttle to Anacortes - ferry to islands.
ACA Northern Tier - Anacortes, Washington to St. Mary, Montana
About 700 miles
https://www.crazyguyonabike.com/doc/?o=1mr&doc_id=2937&v=BxPart II - Glacier/Waterton to Jasper, Alberta
Stunning ride through the Canadian Rockies via Icefield Parkway
ACA Great Parks North - Banff to Jasper, Alberta
The east side route from Waterton via Pincher Creek, Longview & Kananaskis is magnificent
About 500 miles
https://www.crazyguyonabike.com/doc/page/?o=1mr&page_id=26440&v=Nr(Starting at Day 56)
[This would be three good weeks - with VIA Rail train back to Vancouver if you end here.]
Part III - Jasper, Alberta to Vancouver, BC
Lovely ride along Thompson River down to Kamloops - gets busier the further south you go.
You can catch the train from a number of points along the way - it runs every other day, though.
About 500 miles
Part III Alternate - Jasper, Alberta to Prince Rupert, BC
Another option is to ride all the way out on the Yellowhead Highway to the coast.
Some remote sections - massive, spectacular Skeena River Valley
Plus, you can ferry out to Haida Gwaii country
Then fly back to Vancouver and depart or ferry to Vancouver or Washington state
About 675 miles
I would definitely go for III Alternate if there is enough time.
Pic - Mount Robson Provincial Park on a rare, cloudless day