There are many more –er and –ir verbs than –ar verbs

Yes, there is.

"ir" verbs usually indicates on movement (going somewhere). This verb can usually also stand alone and can act a singular word.

example: "Ma', puedo ir a la playa con mis amigos este sabado?"

The girl is asking her mom if she can go to the beach with her friends on Saturday.

"er" usually indicates  present-tense. It cannot stand alone as a singular word the way "ir" can.

example: Saber (to know)

Finally "ar" is a less used verb in spanish. It also cannot stand alone as a word. "ar" and "er" are very simaler verbs, since they both indicate to do something.

Example: Nadar (to swim)

I hope this helps.