On a common time song do lead-ins always start on the 3?

[3ternal]

I've been listening to this lately and seems to be a pattern.

 

[bandcoach]

It can be anything.

1/2 beat, 1 beat, 1 1/2 beats, 2 beats, 2 1/2 beats, 3 beats, 3 1/2 beats

i.e. using just quavers/18th notes bah on-beat quaver, buh off-beat quaver, dah down beat

buh | dah

bah-buh | Dah

buh-bah-buh | Dah

bah-buh-bah-buh | Dah

buh-bah-buh-bah-buh | Dah

bah-buh-bah-buh-bah-buh | Dah

buh-bah-buh-bah-buh-bah-buh | Dah

These pickup beats can use anything that adds up to the number of beats, i.e crotchets/4tr notes + quavers/8ths + semi-quavers/16ths