Yellowstone (the park) is great to see at any time of year. When the crowds are lower, you can slow down and truly appreciate the scenery. The animals in Yellowstone can be harder to find late in the season. There will surely be some around, but it'll be a coin toss for what you'll find. Geyser activity is awesome anytime, especially during cooler weather - the steamy condensation of the hot water is easier to see when the air is cold. Even if you're not a Geyser fan, a trip through the Norris Geyser Basin is worth the time to get there. Same with Mammoth Hot Springs at the north end of the park - worth it to see, and then spend time exploring the Visitor's Center there - see the old pictures, and get a better feel for how Yellowstone came to be what it is - America's First National Park. Old Faithful is always impressive, but after you see it go off a time or two, it kind of loses its "wow factor." It's still great, but is sort of like Niagara Falls: The view never really changes.
. The Old Faithful Inn is right next to the geyser, it's an awesome example of "Parkitecture", and it is simply amazing to see a four-storey high log building. The main fireplace has four sides, and is large enough to walk into. The angled-branch supports holding up every piece of ceiling were hand-picked to match in size and shape as closely as possible. It is a very worthwhile experience.
Grand Teton is a great drive south out of Yellowstone - the GT Park entrance is right outside the exit from Yellowstone, so they're essentially neighbors. The roads are good, and scenery is amazing, especially when there is snow on the peaks. Stop at every turnout to view the mountains, as the view always changes. If you make time enough, and enjoy western- and wildlife-themed art of all types, I highly recommend the Wildlife Art Museum just north of Jackson Hole. (Basically right next to Grand Tetons NP.)
https://www.wildlifeart.org
Glacier NP has a shorter season than Yellowstone, and weather can be very changeable. If the snow hasn't come early, and the Going to The Sun Road is open over Logan Pass, it's awesome to see. If the Pass is closed (which can happen at anytime of the year) it takes more effort to see the whole area, because the park is split into East and West sides, and you have to drive around the outside perimeter to get to those two sides. It's very worthwhile, as is anything around that part of the country.
But it's all worth it, even if you find you'll want to schedule another trip at a warmer time of year. I've been to Yellowstone probably 20 times, once when I camped in an RV for three weeks just outside the park, and I'd go back again tomorrow. As long as you're willing to accept the possibility of weather not being ideal, I'd say go for it. It's very different than anything you have back home.
Have fun!
Dave