With the FiM comics, you've got two series- there's the main series which does 2 or 4 issue story arcs, and one-shots like the recently discontinued Micro Series and the Friends Forever series that replaced it. The one-shots usually focus on just one character in the Micro Series or two in Friends Forever. Volumes one and two have 1 4-issue story arc, three has 2 2-issue arcs, and 4 will have 1 4-issue arc. Issue 19 I got last Friday, but Hot Topic has an exclusive cover and charges $5.50 versus $3.99 for the regular comic.
Also, the comics are still obviously going to be kid-friendly, but they are more oriented toward an older audience (teens, pre-teens, adults) than the show. I don't know how much a 3-year-old would enjoy it. Of course, I know nothing about children.