There is no point in speaking about differences as only the first form is correct (the other two mean nothing).

• on va faire la fête
• on va fais la fete
• on va fait la fete

The second verb in these combinations of two verbs is never conjugated and must always appear in its infinitive form. This combination of "aller" with other verbs in the infinitive is used to express a near future.

futur proche

Unless you are conjugating a verb with an auxiliary, the famous "deux verbes se suivent le second se met toujours à l’infinitif" is the rule, meaning when you have 2 verbes back to back, the second one will be in its infinitive form. If you want to conjugate faire, you must remove the verb aller and then you can say on fait la fête (now), on fera la fête (sometime in the future), on a fait la fête (we did it and it’s over).