Adding an additional same phase? Nothing in the rules?
Short answer: No, you can't.

The rules say: "1 set of 3". So you can't put down 2 sets of 3. The line from the rule states:

"A player may lay down more than the minimum requirements of a Phase, but only if the additional cards can be directly added to the cards already in the Phase."

So in this case, you could lay down addition cards in the set, or cards to extend the run. For full coverage the phase descriptions when you lay down could be described as:

Phase 1: 2 sets of at least 3
Phase 2: 1 set of at least 3 + 1 run of at least 4
Phase 3: 1 set of at least 4 + 1 run of at least 4
Phase 4: 1 run of at least 7
Phase 5: 1 run of at least 8
Phase 6: 1 run of at least 9
Phase 7: 2 sets of at least 4
Phase 8: at least 7 cards of one color
Phase 9: 1 set of at least 5 + 1 set of at least 2
Phase 10: 1 set of at least 5 + 1 set of at least 3  

Which also clarifies the point that each time you go down, you can only complete the current phase you're on, not the next one.