I’ve stayed in all 3 of them.
DT is my favorite out of those 3 - friendly staff, good rooms and a decent fitness/SPA area.
Canopy has a better location if you are a tourist. I just didn’t like the design of the rooms and the vibe of the hotel in general - but some people might prefer it over the DT, which is a classic business hotel.

Garden Inn is simple and good value for money. Pre-COVID there used to be a lot of groups in the hotel, which meant it could get easily crowded, especially at the breakfast. That is probably not the case anymore. Location-wise, it’s close to the DT and located in the business district of Zagreb.

to sum it up: if you are there as tourist and are comfortable with the design/vibe of the Canopy, you might find it the best choice. If the concept of Canopy doesn’t suit you, I would go for the DT, especially as the price difference between the HGI and DT usually isn’t that large.