To get Kanto Starters - since you can't get it from Professor Birch - you can do either of these:
- Go onto GTS. Old generation and early evolution Pokemon like these should result into easy receiving.
- Another trading is trading among other trainers. In ORAS, you can trade with people using either: OR/AS or X/Y. Since you can trade with X and Y trainers, getting it should be simple - if you make a valid trade. (Professor Sycamore allows you to chose 1 of 3)
- Another trading system is "Pokemon Bank, Ties with Pokemon Transfer". You can receive Pokemon from earlier generations if these recent don't give any luck.
- Using a hack like powersave/action replay, should also get you all 3 in 5 seconds - if you have it.
- Using a breeding method to expand your quantity or fill the pokedex - by filling in 9 sections (each one with their distinctive evolutions)
Many ways to get them!
Hope I helped! :)