Hiking the WCT in 3 nights would be tough....that's already an aggressive pace for this technical hike and if you lose the first and last days getting to/from the area then you're really low on time to do it. It's also expensive if you're paying ferry fees, $125 to hike the trail, another $125 for the shuttle back to the start etc.
If you are going to Vancouver Island, the Juan de Fuca trail (immediately south of the WCT on the same coast) is easier to hike because it's shorter (48km vs 75km), much cheaper ($5/day instead of $125) and there is a road that parallels the hike so you can hitch back to the start when you're done, and there are some early bail out options if you're low on time. China Beach to Sombreo Beach would be a nice casual trip in 3-4 days if you're not into pushing the pace. 4 days/3 nights would be decent clip on the JDF because it's somewhat slow hiking, but it would be entirely do-able if you're motivated.
I agree with Stefan that the Howe Sound Crest Trail (HSCT) would be a very nice option. This is my recommendation as well. It starts right at the end of the city, so you can just take city transit and a short taxi ride to the start if you don't have a car. It's a beautiful hike with amazing views of the coast mountains and the ocean. Amazingly, very few people hike this once you're past the Lions. It's only 28km (16 miles) but it's slow going (ie. 1mph average) because the trail is so rugged and undulating on mountain ridges. 3 days/2 nights is about normal, but you could easily spend 3 nights on the trail. If you're thinking of doing this one then I can go into more detail for you. Good fishing at Deeks Lake.
Garibaldi has some amazing scenery and some really nice trails, but it lacks any longer hikes. The trail networks in the different areas of the park do not connect, so it's tough to put together a hike that is more than one night long - unless you're just there to camp. You can put together a 20-30 mile trip, but the trails are mostly so nice that you could hike that in 8-12 hours. I've been meaning to connect all the different networks of trails in Garibaldi with some bushwacking and packrafting to create a long Elfin Lakes to Wedgemont traverse, but I've yet to get it done.