Bicycle Mechanics - chainring question

my bike has 8 gears on the back, 3 gears on the chainring. with the higest gear on the back gear, should I be able to switch to the higest gear on the chainring with out any problem? or do I need to switch to a lower gear on the backgear before I can switch to 3rd gear ont he chainring?

A properly set up drivetrain should allow you to shift into all gear combinations. However, some extreme combos will put a lot more stress and wear on the drivetrain than if there's less deflection of the chainline. For triples, I generally try and avoid the last couple of taller gears of my cassette when in the inner chainring and the upper two or three gears of my cassette when in the outer chainring. If I'm in the middle ring, the whole rear is fair game although the very highest and lowest cogs are probably undergoing some pretty heavy wear even in that case.

There is a fair degree of overlap in the 3 gear ratios. If you are in the middle ring on the smallest rear cog, then shift to the large ring, it is quite a jump. There are usually a few intermediate ratios if you also shift to one of the larger rear cogs.
Similarly, when you shift from middle to small, you will find some intermediate gears by moving to a smaller rear cog.
Whether you shift the rear before or after the front is your decision, but I would try before if you can. Shift one or 2 cogs towards the middle of the cluster before changing rings.