The problem you will have is this, each section of your yard will probably have different water requirements. If on area has full sun it will have a different transevaporation rate that of an area with partial sun or complete shade. It will also depend on the plants you are watering.
Usually a professionally designed system will take all of this into account.
There is probably a reason it was designed as 3 zones.
Simply wiring everything together is most probably a bad idea if your system was designed correctly in the first place.
