I can't seem to find the three other critters. I know one of them is a polar bear.
The bear is not in the game yet . The same goes for other 2 critters. They will probably appear in act 3 or 4 tho . Just guessing:P. Btw if you wanna know how those 3 critters look like. Just check the official forest of love site https://www.theforestoflove.com/ and click on meet the critters.Also when you get to the critter cards click on them to move them :P(this will make sense once you get to the cards with the critters).
New information.The bear might be in the patreon version of the game (probably). I actually have no idea .For more information tho check this site https://theforestoflovecharacters.carrd.co/#hints .Hint : scroll down to number 25 on the linked site :P.