how easy is it to jump on the M3 from heathrow?...it will be my first driving experience in the UK in over 20 years...and of course back then i never drove near London

Get good directions from the rental car place. Driving on the highway, because it's divided, is easier than smaller roads.

There are a number of threads here with tips about driving in the UK. You might want to have a look.

Simple. Heathrow is roughly at the junction of the M4 and the M25. Take the M25 soutbound (anti-clockwise) and you'll eventually meet the M3.

However be prepared for heavy traffic on the M25 at certain peak hours (inlcuding weekends - Sunday afternoon can be a nightmare).

Kate is right. The Heathrow spur road will take you directly onto the M4.
A couple of things to remember about driving in the UK. Firstly the roads (especially motorways) are extremely busy. I found it easier to drive in LA when I was there last year, although I found trying to squeeze through the Lincoln Tunnel in NYC a couple of months ago very similar to driving in British cities (even the smaller ones!).
Secondly, remember your lane control. It is an offence to overtake on the inside lane of any car. In a similar vein, once you have overtaken you should pull back into the slower lanes.
Lastly, the UK has become Orwell's "Big Brother" with speed cameras everywhere (and I mean EVERYWHERE!) . Cameras in 30 mph speed limits may be set to trigger at speeds of about 35-36 mph. Basically, know your speed limits and keep to them. A car rental company WILL pass on any fine to you.

thanks...good info to remember...i will be careful

Although passing on the left is illegal, it doesn't mean it never happens! So check your mirrors when you are changing lanes.