1. Legal notice can be sent to one person only once and not repeatedly even though he has not responded to the first notice.
Now the legal demand notice can be sent to the other two persons too, but since you have already sent a legal notice to one party, it would be better if you send a rejoinder notice to the same party and implead the other two parties in the rejoinder notice, this will clear the ambiguities, if any.
2. The time to be given for their compliance is your own discretion.
3. It is better you implead everyone as parties to the suit and file a money recovery suit, because sometimes due to technical issues, the summary suit may not be maintainable.