Sounds like you don't know what you like and what you don't well enough. Go read things. Short stories, flash fiction, long series, fanwankage lemon, whatever. See films.
Try this trick: write a list of things that will NOT happen in your story. It can help.
And if that don't work, you only have an element, not an idea. So add elements. Make the triplets ninjas. Make one of them mentally unstable. Make them have a deadly secret. Make them magical. Make them love the same girl. Make them have a donkey they can't get rid of. Shamelessly rip off tiny parts of stories you have liked or hated. Give them a Blessed with Suck
magic ring, a cat that can sense when things are "wrong" but cannot communicate more than a normal cat, force them to leave their homeland for the bigger good, put a sniper after them, make them poor in a city of rich Beautiful People
, question their identity on a cultural scale, make them face the realities of guerilla slave freeing.
Don't be afraid of things being contrived, cliched, or boring: pay thought to ALL kinds of combinations of elements, and if something feels "wrong" then write it on your list of "not going to happen", with some basic reasons for your judgement. "Orphaned ninja triplets know a deadly secret". Then analyse those reasons. Then try to come up with something you still like about the concept. It can be anything, from a whole concept to a sliver of a feeling. "Orphaned ninja triplets know a deadly secret" could be good for "orphanhood: no ties to family, maybe society cast-offs?" or "secrecy + deadly job sounds like plot fodder"...
The more "stupid" ideas you analyse like this the more workable idea matter you will have.
Have one hat (A) full of paper slips doing with actions, jobs, positions and characteristics: such as "crime-solving", "house-keeping", "unemployed", "overworked", "secret agent", "teenager", "mutant", "ninja", "turle", "clan traitor", "Nazi equivalent", "insurgent", "magical girl" etc...
Have one hat (B) for surroundings, culture, milieus, genres: "in space", "in mythical China", "in barren wastelands", "in a steampunk world", "among the mob/mafia/yakuza", "among the parias", "in a forgotten temple", "from Utopia City", "from the countryside", "from a secret colony on the Moon" etc...
And one hat (C) for random story elements, plot twists and such: "but zombies", "with annoying talking item", "but the reward was a lie", "but their father is a criminal", "while falsely accused", "but are made an offer they can't refuse", "but the universally adored religion was a commercial-political ploy", "but the magic item affects personality to advance its own agenda", "but the dreamboat man is their brother", "but it was just a dream and the actual situation is much more...", "while fleeing a horde of bounty hunters after their heads", "while smuggling an important item", "while looking for the wish granting magical balls scattered all over the world" and so on...
Draw two of each, and make a sentence:
AA triplets BB try to find their mother, CC.
Figure out what you don't like and what you like. Write things down. Repeat.
Oh, and one thing to work on before you begin: figure out why your triplets need to be triplets, not just ordinary siblings born in a small frame of time. Also come up with their motivation to find their mother. Come up with crazy reasons, petty reasons, choose some nice ones that give you ideas, and please don't make it "because she's their mother duh).