First of all, you need 120Volt line to operate timer.
Or a neutral wire.
Typically most buildings with 3-phase also have ordinary 120Volt outlets.

Then wire timer to 3-pole contactor to control 30 amp 3-phase load
http://waterheatertimer.org/How-to-install-3-phase-timer.html

Selecting timer:
1) Timer clock motor needs to be 120Volt. (or if you buy ET1715 timer, or other timers, they can operate on 277Volt by changing input voltage selector as shown in manual,
or other change as shown in manual. 
2) Timer can be any load rating since it is only opening-closing 7amp contactor coil.
3) If you want 7-day programming:
To choose timer, it's good to understand timer programming.
Each timer offers different number of programs. And different level of programming complexity. 
The more programs, usually the more complex the timer.
When setting a program, each program has ON time and OFF time.
And then you choose day or days for that program to run.
Suppose you want timer to turn ON at 8am and OFF at 5pm for Monday thru Friday: that would be 1 program.
Suppose you want timer to turn ON at 8am and OFF at 11:59am Monday thru Friday, and then turn ON after lunch at 12:59 pm and OFF at 5:00 pm Monday thru Friday, that would take 2 programs.
Then suppose you want timer to operate Saturday: On at 8 am and OFF at 1 pm, then that would take 1 more program
Each ON and OFF setting uses 1 program. And you can pick individual days or multiple days for that program to operate.

Three example timer choices:
a) Intermatic EH10 120Volt programmable timer has 6 ON and 6 OFF programs per week. Set times down to 1 minute accuracy.
http://www.1000bulbs.com/product/3398/ELEC-EH10.html
http://www.amazon.com/Intermatic-EH10-120-Volt-Electronic-Heater/dp/B000GHIPOA
http://waterheatertimer.org/pdf/eh10-manual.pdf

b) Intermatic 1715C timer has 14 ON and 14 OFF programs per week. Set times down to 1 minute.
Timer clock motor is default 120Volt, but can be converted to run on 208/240/277 volt using instructions in manual
http://www.amazon.com/Intermatic-ET1715C-30-Amps-Electronic-120-Volt/dp/B003A45RCY/ref=sr_1_cc_1?s=hi&ie=UTF8&qid=1324175390&sr=1-1-catcorr
http://www.intermatic.com/~/media/files/intermatic/products/instructions/timers/et1715c%20-%20english.ashx

C) Intermatic GM40W mechanical 7-day timer.   
Minimum ON-OFF settings 1 hour and 45 minutes.
Mechanical dial rotates once per week. EZ programming by moving tripper in or out. No worries teaching folks to program timer.
http://www.amazon.com/Intermatic-7-Day-Outdoor-Timer/dp/B003A3TXW0/ref=sr_1_1?s=hi&ie=UTF8&qid=1324175683&sr=1-1