I have the same set up with the 3 dot scope, you can use it 10,20 30, yds or 20, 30, 40 yds, hope this helps,

 

so if i sight in the first dot for 20 yards then at 30 yards i just use the second dot?

 

Basically, yes. But you have to shoot it to see if it actually is on at 30yds. It could be on at 28-29yds, or, 31-32yds depending on your setup.

 

It will depend how heavy of an arrow/tip you use... I have 4 lines on my scope, and because I switched to 125gn points, my scope is sighted in at 20yds, 27yds, 33yds, and 40yds for the 4 lines...

If I go back to a 100gn tip, the second line will shoot closer to 30-32 yards because it will drop less due to going faster!!

 

i have a single crosshair scope on my crossbow for now so i might just stick with that.

 

If your first one is at 20 yds your second or middle one should be 30 yds and the bottom one should be 40 yds, site in for 20 then step out to 30 yds and you should be on at 30,yds, and the same for 40 yds, I set mine up this way, my is 10,20,30 because all my deer are close, but start at 20 and then step back to 30 to see were your at,

 

This video may help. The real question is what distance to set the first dot for -20 or 30 yards?

http://www.youtube.com/watch?v=g_WOWsdGQgI

 

Some scopes, such as mine from Tenpoint, It has a speed setting dial on it that you would adjust to the crossbows approximate feet per second shooting. With mine set to that and sighting in with first dot at 20yds the second is dot is on at 30yds, the third is at 40yds and fourth dot is at 50yds. With sighting in at 20yds, and experimented at longer yardage and little tweeking the scope was true to course.

 

The 'speed settings' are no more that magnification changers of a variable scope. Increasing the setting DECREASES the distance between reticule aiming marks, decreasing does the obverse. The weight of your arrow and it's speed determines it's trajectory i.e. where it hits at 20, 30 etc. So, once the 20 yard zero is set, you may have to play with the "speed ring" to get the desirable point of impact (POI) at other distances. It's not uncommon that the POI won't correspond exactly at 30,40 etc yards given difference in the two underlined variables. You may be a bit low at say 40, so changing the setting, or setting the 20 yard zero a tad high, may do the trick. I generally set my 20 yard and 40 yard to come as close as possible at 30....my self imposed ideal range.