Your first opportunity to earn this achievement will be on the second level of the game, "Jungle Rollers".
You will need to collect character tokens. Character tokens will be found in crates marked with a question mark (?) and when the crate is broken, the token will be collectible with a face on it (similar to an extra crash life).
Once you find the third character token, you will be granted access to a portal that leads to a bonus area. Generally the third token of a character set is close to the portal entrance.
This will likely be one of the first achievements you unlock, and you'll likely unlock it with no issues. However, if you do have issues, here's a quick video I found that outlines how to 100% the second level "Jungle Rollers". The character tokens are shown.
[Credit to xTGE for the great video!]