Oooo....this is a difficult question!
Whenever we think of longbows, we always end up visualizing the battles of Crecy (1347), Poitiers (1356), and....of course....Agincourt (1415).
But England also used longbows against the Scots, most notably at the battle of Flodden Field (1513).
Now from the English point of view, all these battles appear to be defensive, with the English stationary while the French or Scots are shot to pieces as they tried to advance.
But let's for a change look at the situation from the French or Scottish point of view:-
The English have advanced and then stopped some 300 yards/metres/paces away....
They begin to loose volley after volley of arrows onto your forces...
What are you going to do?
You only have three choices:-
1) withdraw and leave the English in control of the field?
2) stand there and take it, while not being able to reply?
3) or attack and get shot to pieces at close range!
So it
FORCED the enemy to attack or run.
So yes....as Yoda said, in Civ3 they should have an attack of say 4, a defence of about 2, and a defensive bombardment of at least 4 (I would also give them a rate-of-fire of 2, as they could loose a dozen volleys a minute compaired to a musket's 2 or 3.
(
BUT, they should be expensive....it takes years to build up the muscles in order to pull bowstring of a longbow, while any weedy little bloke with only a week of training can use a musket.
)
This way, if you ever have to face an enemy armed with longbows in the game, then
YOU will have the same choices that their opponents had in reality....
....either try to attack them,
OR, avoid them!