Giving up one spell list after 7 levels of it to start another class with spells starting at caster level one of only the slots of a first level character for that class again is usually not a good idea. A bard 3/magus could have a few 1st level bard spells or a magus 10 have have a 4th level spell plus one or two spells of 1st/2nd/3d, all at full caster level. You better be getting really kick ass class abilities to justifly the loss of signifigantly more powerful spells with full caster level. The bard would not even increase your bab, doing 7 levels of magus could get you another bab+1 at magus 8 but instead as a 1st level bard, you get +0bab and fall behind even other single clessed PC who stick with just one 3/4 class. Maybe do eight levels of magus to get that extra bab and. Boost to both fort/will saves before looking for another class or prestige class.