If you are using a custom skin, it will be because the creator of the skin has made them invisible on purpose. This is because hit300s are usually very very distracting while playing the game.
If you want the default hit300s back, you could open the osu! folder from the settings -> Skins -> click the skin folder you are currently using. You can then move out/delete all of the files that say hit300 or similar, go back into osu!, and press ctrl+shift+alt+s to reload the skin.
If you want some custom hit300s that aren't invisible, you'll have to find them from another skin, or somewhere online (will likely find these in older skins when people used to use hit300s, or on skinning websites).