The only requirement is that at least 3 of the "drawn" cards must be for Major Raids. If there aren't 3 in the initial drawing of 10, keep drawing until you get to 3.
What you do with those raids is entirely up to you. You are not required to actually perform any raids. Any raids you do select can be placed in any time slot (assuming no morning haze, with a max of 3 per time slot) and within that time slot, raids can be performed in any order. And even though you must have 3 Major raids available, there is no requirement that says you must use them.. You can have all minor raids if you so choose.
And that is why Eagle is so much fun!