Typically, if an online slot game has 3 reels, it is called a “reel” slot. If it has 5 reels, it’s a “video” slot. This is how I split them up, anyway. This is how most online casinos seem to split them up as well.
There are gray areas and exceptions to the rule, such as the Playtech game Gold Rally, which has 9 different spaces on a 3 x 3 grid… like a tic-tac-toe board. I call this a 3 reel, 8 payline “reel” slot, but some might say that this game actually has 9 unique “reels” and 8 paylines, and they might call it a video slot since it doesn’t really look much like a traditional reel slot.