IMO, no. Classes like the magus have abilities that come on at certain levels and pushing them out seriously hampers the overall concept, regardless of whether it is a STR or DEX build. Particularly 3 levels.
That said, are you really looking at going all the way to level 20? I'm at the point I look for things that trade out the capstone since I don't ever expect to play (even as a GMPC/NPC) anything that high of a level. Getting rid of ANY capstone for some great feature earlier is a win-win in that case.
haha, if they are your last three levels then maybe.
okay first off, yes it can work, your character will likely still be viable and playable, it's just likely a decrease to your power. But if you want to go for it, have fun, you'll likely fine.
Rule of multi-classing, maybe good if you want to hit things, not good if you plan on having the magics.
Like let's take this magus.
a magus 4 we'll refer to as magus vs a magus 1 rogue 3 we'll call him rogue
Both have dex to attacks but the rogue has 1 less bab -1 accuracy rogue.
magus has 4 lv1 spells and 2 lv2 spells, rogue has 2 lv1 spells, big bonus magus.
damage wise rogue has dex to damage (let's say 5) and magus has his non-dumped str of 0. damage without spells goes to rogue. But that's just this level, until now both are running off of the 0 str. A magus will likely buy an agile weapon for dex to damage soon (or be a dervish to already have it).
rogue cannot spellstrike, when magus spellstrikes with shocking grasp he deals +4d6, with chill touch (I think that's the right name) it's 1d6+4. Both do more damage than the rogues normal attacks.
Rogue has 2d6 sneak attack if set up correctly, this counters ~7 damage from the advantage the spellstrike has making them pretty equal.
So you're less accurate to be about the same on damage, unable to spellstrike and behind 4 spells per day. Doesn't seem like the best of trades.