From the little bits I know about this, I'd say using 3 of every element is a bad idea, using some wiki data as an example.
Ice to Fire = 200% Damage
Ice to Lightning = 150% Damage
(100%) Ice (Magic) to (100%) Ice = 0% Damage
Fire to Ice = 200% Damage
Fire to Lightning = 150% Damage
(100%) Fire (Magic) to (100%) Fire= 0% Damage
Lightning to Ice = 150% Damage
Lightning to Fire = 150% Damage
(100%) Lightning (Magic) to (100%) Lightning = 0% Damage
(Same elements reduce eachother by 11.1% per level, 9 Levels = 100%)
So let's use your idea, being 3 of each element, and putting you up against an element of the same level (3).
So Damage taken goes:
Ice damage modifier:
From Ice = 3 x (-11.1%) =
-33.3%
From Fire: 3 x 11.1% =
33.3%
From Lightning: 3 x 5.55% =
16.65%
Fire damage modifier:
From Ice = 3 x 11.1% =
33.3%
From Fire = 3 x (-11.1%) =
-33.3%
From Lightning = 3 x 5.55% =
16.65%
Lightning damage modifier:
From Ice = 3 x 5.55% =
16.65%
From Fire = 3 x 5.55% =
16.65%
From Lightning = 3 x (-11.11%) =
-33.3%
In conclusion:
Damage taken from Ice =
-33.3% +
33.3% +
16.65% =
16.65%
Damage taken from Fire =
33.3% +
(-33.3%) +
16.65% =
16.65%
Damage taken from Lightning =
16.65% +
16.65 +
(-33.3%) =
0%
So you'll be weaker to Ice and Fire while your resistance to Lightning will remain unchanged.
HOWEVER, this is just an example and actual numbers depend on the elemental levels of both you and your enemies.
But it is a good example to show that you can't just increase your resistance to every element without a penalty of some sort.