Let A1 have the vehicle's last PM date
In another cell use =IF(TODAY()-A1>90,"PM needed","")
Or use this formula in a conditional format (Format|Conditional Format) to
make the row red.
If B1 has mileage of last PM and B2 has current mileage =IF(B2-B1>2999,"PM
needed","") will work
Also =IF(OR(TODAY()-A1>90, B2-B1>2999),"PM needed","") will cover both
best wishes

Do you track the mileage daily?  If you don't keep track of the mileag
somehow, to where excel knows the vehicle has reached over 6000 mile
in an interval, it won't work exactly, you would be estimating dail
mileage, and that's never worthwhile IMO.  

My suggestion:

Create a column for last date of service for the vehicle, and th
mileage at the time of last service, current mileage, and obviously
vehicle identifier of some sort.  If you update this daily, then hav
one column that holds the current date, and change it daily. (there ma
be a formula that holds current date, but I am not positive)

Place your conditional format on both the date and mileage fields a
follows:

For date:
If cell value (of last date of service) - is less than - (current dat
cell) minus 90, and format as you please.

For mileage:

If cell value (of current mileage) is greater than - (mileage at las
service) + 3000, then format as you wish.

Let me know if that would be satisfactory