Is it possible to have more than 1 spiritual weapon in existence at one time?
If I have 3 lvl 2 slots
Memorize spiritual weaponx3
Have 3 round casting deployable flash mob?
Yup. Nothing in the spell description saying otherwise. You can summon several of them and then go 'maces up, maces down' on a target if that suits you. Can be useful for those days when the shadows and wraiths are coming to get you.
You can cast as many as you want, but remember that you have to rediret them to a new target with a move action once the old target dies and you have only 2 move actions per turn (if you convert your standard to a move).
So either you plan tactically, so the enemies all die on different turns, or you will waste turns of your spell.
Also, they are only rounds/level, so keep in mind that your "flash mob" will not exist for long.
Other than that, you're golden.