A general answer for this is almost impossible. Are all of the PC's getting the template, or only some? If only some, then the LA depends on how the other characters generate stats and hit points. It's much stronger if you use 15 point buy than if you use 30 point buy.
The baseline stat array is 15-14-13-12-10-8. Compared to that, the stat modifiers are +2/+2/+3/+3/+4/+5. Let's say a typical class has 2 primary stats, 2 secondary stats and 2 stats that can mostly be dumped. For a typical fighter, Str and Con are primary, Dex and Wisdom are secondary, Int and Charsima can be dumped.
The template gives +2 to the primary abilities (over what you would get before) and +3 to the secondary abilities. The bumps to the dump stats don't matter so much, other than they make open up feats like Combat Expertise that require a stat that's not otherwise useful to the character. It's probably effectively about a +10 to ability scores, not quite arranged in an optimal manner. I'm not sure if there are rules for that, but it seems like that's worth about +2 LA.
Maximum hit points is like +6 to +12 to Constitution, but only for purposes of calculating hit points. I'd call that another +1 for a total of +3.
Sanity check: does that make sense?
For melee types, 3 levels would get +3 to hit, +1 to +1.5 to all saves, 1.5 feats, 3d10 + 3*con hit points, +3 on skill checks and 3 levels of class features. The template grants +2 to hit and damage, +3 to saves, +(2-3) * level hit points, +(2-4) on skill checks and 2-5 extra skills.. Hit and damage is about equal, skill checks are about equal, saves are better, hit points are worse until somewhere around level 10. That still leaves the feats and class features to balance against the extra skills. +3 is probably on the high side, especially since PF classes have so many nice extras.
For caster types, 3 levels would get +(1-2) on save DCs, +3 on caster level checks and 1-2 more spell levels and similar to the melee types on hit points and skills. The template gives +2 to save DCs, some bonus spells (probably of lower level) and similar to the melee types on hit points and skills. The spell DCs are about equal and the hit points balance sooner (since the lost hit dice are smaller). The lost caster levels and spell levels hurt a lot.
At +3, I'd take it if I was building a skill monkey. I might take it if I was building a fighter type. (Fighter, Ranger, Paladin, etc.) I wouldn't take it as a caster, even at +2.
So +3 is probably right, and, like all level adjustments, any caster who takes it will be weak for his level. (If you think casters are still overpowered, this could be a nice way to compensate.)