Help

I'm writing a book and it is a western novel! I'm not gonna give out too much, but my main character is named Peter (19) and there are two other characters in the book that are whom i believe to be secondary main characters. their names are Maisie (21) and Banks (27) I'm writing in 3rd person, and whenever i write, i seem to switch from focusing on each of the three for a few actions for a time. 

for example, it starts with peter here on one side of a room, looking at something and fiddling with it. once nothing in particular is happening, it would switch to Maisie digging through a trunk. once Maisie found nothing, i would switch to banks climbing up into the hayloft. 

is this a bad writing practice? I'm not sure if its a habit i should break or if  it makes peter, maisie, and banks all main characters or no.