Your three S corps are considered the same employer for one thing, so you can't double up or triple up for that reason.
Also, employee elective deferrals are per taxpayer. So you contribute $13K in total to all of the Simple-IRA plans you have.
Finally, you may want to talk with your tax adviser about simplifying your reporting structure. It may make sense to make your three S corporations "qualified S corporation subsidiaries" or QSUBs under one parent S corporation. That would mean one 1120S return, one payroll service, one pension (which is basically what you need anyway), etc.