Question

What is the difference between soft-boiled, slack off and recover?

Answer 1

1. Soft-Boiled

   Soft-Boiled was introduced in generation 1.
   Prior to Generation VII: in the field, using Soft-Boiled will take away 20% of the user's maximum HP and give it to another Pokémon. (It is unusable if the user's HP is below 20%.) (source)
   Only chansey and blissey can learn Soft-Boiled.
   No Pokemon can learn Soft-Boiled as an egg move.
   Soft-Boiled has 10pp.

2. Slack Off

   Slack Off was introduced in generation 3.
   No Pokemon can learn slack off as an egg move.
   Slack Off has 10 pp.

3. Recover

   Recover was introduced in generation 2.
   wooper, quagsire, sableye, lileep, blipbug, dottler and orbeetle can learn recover as an egg move.
   In generations 1-3 recover had 20 pp while in the generations after that recover has 10 pp.

I'm pretty sure that's it.

Answer 2

In generation 3 and earlier, Recover has twice as much PP as the other moves.

In games where field moves exist, Softboiled can be used as a field move, paying some of the user's own HP to restore an equivalent amount of HP to one of its teammates. Milk Drink, another similar move, can be used the same way.

Other than those, and the different pool of Pokemon that can learn each of the moves, they're no different.