It is common for original equipment manufacturers of package air compressor units to size the motor such that it must run at the service factor. I do not agree with the practice, but it is what they do (to save money). In your case it is not as bad as some, a lot of times you will find these motors running at the full rated service factor. A 3% overload should not cause a major reduction in your motor life based on the 1.1 (1.15?) service factor. My posts tend to be somewhat long so there is my answer in a nutshell. Read on if you want some more info on the subject.
With respect to the insulation temperature classification and allowable temperature rise ("motor construction is designed based on class F insulation (IEC 72 & 72A) use to Class B temperature rise"
, this combination is standard for industrial motors with a service factor of 1.15. The Class B temperature rise indicates that at nameplate HP the temperature rise will approximately 80C. Added to the rated ambient temperature of 40C, these ratings indicate a total temperature of 120C and a hotspot temperature of 130C at nameplate HP and Voltage. The Class F Insulation temperature classification allows for a maximum allowable hot spot temperature of 155C. This is what allows this design motor to operate at the service factor without exceeding the temperature rating of the insulation. The additional 25C of allowable temperature rise equates to the ability to exceed namplate power (or current) without overheating the insulation, which in turn equates to the service factor rating of 1.15.
Will you reduce the life of a motor by operating at the service factor? Based on insulation life, the answer is no. Motor insulation service life calculations as a baseline use the case where the temperature rise is equal to the temperature class at nameplate power, ie. a Class B temperature rise with Class B insulation (ie. rated for 1.0 service factor) operated at 1.0 service factor (nameplate HP). The average insulation service life under these conditions is 20-25 years. A motor with Class F insulation and a Class B temperature rise ( ie. rated for 1.15 service factor) will have an insulation service life 6 times greater if operated at 1.0 service factor. When operated at a 1.15 service factor though, it will have a temperature rise equivalent to Class F and the insulation service life will be reduced back to 20-25 years. This is because as a rule insulation life doubles for every 10C reduction in temp and halves for every 10C increase (up to insulation temp rating, after that it drops off pretty quick). So yes, you could say that the motor insulation life will be 1/6 of what it would be if operated at nameplate HP or 20-25 years versus 120-150 years.
However, if you are thinking that many motors last 20 years but none will last 120 years no matter what then you are correct. Bearing failure, contamination, voltage problems, or overloading will inevitably limit the total life of the motor. The oldest induction motor I have seen continously running on an original winding was a 1942 unit. It had a bearing failure and although the winding tested ok, a visual inspection determined that the original glass insulated wire was in very poor shape due to oil contamination.
So, in sum, you could expect an full insulation service life of 20-25 years life out of a motor operated at the rated service factor whether it is 1.0 rated or 1.15 rated. In reality though, you will never see it. The problem with this practice is that you have no room for error since the motor is operating at its' maximum temperature. If you do have a voltage problem, an overload problem with the compressor, or a cooling problem then your motor will certainly burn up. A motor with a 1.15 service factor rating operated at 1.0 service factor (ie nameplate HP) would have a greatly increased chance of riding through such an event without damage due to the additional temperature capacity of the motor.
