[Magrus]

Well, on an average day unless you have advance knowledge of a specific threat coming along, here's what I would memorize.

Level
0-Detect Magic, Read Magic, Light, Ray of Frost
1-Protection from "--", Mage Armor, Magic Missile, Burning Hands, Grease
2-Elemental Dart, Bears Endurance, Spider Climb, Protection From Arrows
3-Haste, Protection from Energy, Fireball, Dispel Magic
4-Stoneskin, Spark Shield

That would give you stuff to protect you (Protection from "--", Mage Armor, Protection From Arrows, Protection from Energy, Stoneskin) a way to harm other people when they hit you (Spark Shield), a spell to boost your Hit Points and Fortitude Saves (Bears Endurance), damage spells (Magic Missile, Burning Hands, Elemental Dart, and Fireball), a way to move faster and attack more often (Haste), a way to increase your movement options (Spider Climb) and a way to reduce those same options for your opponents (Grease), along with a way to dispel spells other people cast (Dispel Magic).

If you end up knowing your going to be facing a specific opponent later with a weakness, say they are weak against fire, or use a lot of say, acid spells and attacks you can change up your spell list the next time you rest. If you know you need a decoy, you can pick one of the summon monster spells too.

For the 3 spells you choose that you know well, I'd personally go with Stoneskin, Protection from Energy, and Elemental Dart. That would give you a flexible attack spell, a way to protect you from melee attacks, and a way to protect you from spells and elemental damage.

You can cast a lot of those protection and "buff spells" like haste and stoneskin on DZ and vice versa too.