Figuring spoke length
 

I have a VeloOrange touring hub, 135mm, 36 hole , 60mm spoke flange diameter, and considering building up a wheel w 26" Mavic EX721 rim 3 cross. I've gone through three different on line spoke length calculators and am coming up with three different length results varying by 3mm. If on the far off chance someone has built this combo or found a particular calculator accurate I would appreciate the feedback.

There are a couple of possible culprits, the most probable one being that if you picked the numbers out of a chart, the different calculators may use different ERD values. Some use the value where the nipple meets the rim - which would leave your spokes a tad short, others add a few mm to get full penetration of the spoke into the nipple.
Some take spoke stretch into account if you enter a thin spoke etc.
The recommended way is to buy the rim, and measure the rim to where YOU want the spokes to end up, and use that number. You really shouldn't be seeing 3 mm difference for the same calculations using the same input.

This. Measure rim and hub yourself. I aim for spokes to get to the nipple screwdriver flat, and use spocalc spreadsheet to calculate spoke length. 4 wheels so far, and their numbers work great for me.

I can't stress enough how IMPORTANT measuring everything on your own is. That is if you understand how to measure... I work in optics manufacturing, metrology is no big deal to me. It might not be as easy for some, I'm not picking on anyone, I'm just stating the facts.

Measure it, calculate it, and you will probably see the same numbers spit out :)

Using this calculator I came up with 255 and 253. Spocalc, by Damon Rinard. A free Excel spoke length calculator.
Jobst Brandt's book has the formulas in it so that you can figure your own.

X2

Measuring it yourself is really important if you want things to be spot on, it is safe to say ERD's can have a tendency to vary even with the same rim purchased at the same time.

Thanks all, I shall measure

The reality is that even if you enter identical data, various spoke calculators will give you slightly different results.

The basic formula that most use will return the end of the spoke to the ERD you entered. However, there are some fudge factors not in the basic formula and the various programmers handle these differently with adjustments. There's the allowance for spoke elongation under tension (or not). There's the added length needed to allow for the effect of basket weaving the spokes at the last cross, and as flanges get large and you have crosses that put the spokes out away from radial, there's some added length not factored in the basic formula.

So for these reasons alone, the various programs will give you different answers. Then there are assumptions made a desireable margin of safety so you don't run out of thread, or so spokes don't end up short. Different builders have different biases here, and so do the programmers.

So my advice is to always use the same program so you have good sense whether it errs high or low and by how much. Once you're used to a program, and enter consistent data (measure yourself the same way), you'll add or subtract your own correction factor and hit the length dead on every time.

Since the OP is doing it for the first time, he should use identical data in 3-4 programs, and go with the weighted average. That should be close enough, and he'll then know how to adjust future calculations.