My sorcerer has 4 CL 3 scrolls of Fireball. I know that he cannot use them before deciphering them and this can be done in advance.
Can he decipher all of them before a battle? How long before he would need to decipher them again?
First off, CL 3 scroll of fireball is a very strange occurence, since the scroll can not be written at a caster level lower than a minimum required to cast a spell in question, which is CL 5 for a 3rd level sor/wiz spell.
Second, deciphering the scroll is a full-round action which can very well be done in advance.
Decipher the Writing: The writing on a scroll must be deciphered before a character can use it or know exactly what spell it contains. This requires a read magic spell or a successful Spellcraft check (DC 20 + spell level). Deciphering a scroll is a full-round action.
A character can decipher the writing on a scroll in advance so that she can proceed directly to the next step when the time comes to use the scroll.