You cannot target yourself using party#. You use "target=player" to target yourself. For each person in the party other than yourself, you have the options of party1, party2, party3, and party4. There isn't a party5 targeting option. All of these targeting options are for the other players in your group. Party1 is always the party leader.

Also, if you're creating macros to heal specific party members, then using /tar party# is not necessary. This is especially true if your using shamans/druids as DPS who also heal. If you switch targets to heal, that means you have to switch back to start DPS'ing again. Instead, you can include your desired target in your macro. For example your original macro was:
/tar party3
/cast [mod:alt] Chain Heal; [mod:shift] Healing Wave; Lesser Healing Wave

This could be re-written to:
/cast [target=party3] [mod:alt] Chain Heal; [mod:shift] Healing Wave; Lesser Healing Wave

This would allow your shaman to keep his/her original target so that they could resume DPS as soon as the heal finished without having to reacquire a target.

A full set of party healing macros would look like:

/cast [target=player] [mod:alt] Chain Heal; [mod:shift] Healing Wave; Lesser Healing Wave
/cast [target=party1] [mod:alt] Chain Heal; [mod:shift] Healing Wave; Lesser Healing Wave
/cast [target=party2] [mod:alt] Chain Heal; [mod:shift] Healing Wave; Lesser Healing Wave
/cast [target=party3] [mod:alt] Chain Heal; [mod:shift] Healing Wave; Lesser Healing Wave
/cast [target=party4] [mod:alt] Chain Heal; [mod:shift] Healing Wave; Lesser Healing Wave

If you're using a dedicated healer whether it be a shaman or druid (or any other healer) then it isn't a big deal to use the /tar party# line in your macro. But it is really going to slow down your DPS output if you're DPS'ing and Healing with the same character.