Of the three, Mobility is the hardest to figure out how to use. My suggestions...
You can use Mobility actively on a heavily armored character by seeking to provoke AoOs. Combat Reflexes is rather rare in low level play, so this opens up holes in the opposition's threat zones. Particularly useful when trying to force your way through a door or tight space.
Another use of Mobility is when purposefully provoking an AoO through movement, so that you can more safely cast a spell or drink a potion. The +4 AC is not a guarantee of safety, but it helps.
A nice trick with mobility is to move such that you provoke an AoO, say, into a flanking position. Instead of taking the normal attack, grapple instead. This is particularly useful against high AC spellcasters.
Spring Attack is quite straightforward.
The classic hit and run allows you to make an attack without suffering an AoO from your target. If your mobility is very good, you may get a flanking attack without leaving yourself vulnerable to being flanked yourself. Combines well with Tumble.
Spring Attack is a near necessity against monsters with Reach. Creatures with inherent reach are big, and big creatures are strong. So provoking an AoO charging in isn't an attractive choice. If you don't have Spring Attack, a tough fighter can just wait with a Ready Action for the enemy to engage him. The disadvantages of allowing the opposition to choose when to fight should be obvious.
Whirlwind is great when and only when you can get the set up. It depends a lot on the style of your campaign -- in many campaigns Whirlwind is nearly useless. Remember that a regular old full attack lets you get iterative attacks and also allows you the option to hammer a weaken foe dead. If you are regularly surrounded by enemies, they are probably weak and Cleave or Great Cleave will serve you just as well or better. Whirlwind is only a big advantage when you are surrounded by medium strength enemies. Does that happen much? Not really.
