combining modules

hi ! iam new to arduino .. my project has 3 modules

  1. read pulses from electricity meter and send them as message through gsm modem
  2. home appliance control through gsm commands
  3. whenever there is a call to gsm modem it should send electricity units to mobile
    i have written codes for each of these but iam facing problem in combining them
    how can i combine them ?? the frst one should run continously second one should act on a message to gsm and third one should act on a call to gsm

Have a look at this simple merge demo.

If you have used the delay() function in any of your programs you will need to change to using millis() to manage timing as illustrated in several things at a time.