It will depend upon how large your copy stand is and the size of your subjects, plus how much magnification you need.
I'd look at the EF 50/2.5 "Compact Macro", EF-S 60/2.8 Macro, and EF 100/2.8 USM (possibly the 100L IS, but the stabilization would be more useful for handheld work, off the copy stand).
For indoor, tabletop studio work with a crop sensor camera (like your T3, but a different model), I often use the Canon TS-E 45mm. It's not a true "macro" lens, but quite close focusing, and can be used with macro extension tubes for higher magnification. The tilt and shift features are useful with three dimensional subjects, allowing the plane of focus to be carefully aligned or the lens to be offset to "dodge" reflections. This is a manual focus only lens, probably doesn't matter for copystand work... but you might not need the TS-E features, if your subjects are primarily flat.
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Alan Myers
San Jose, Calif., USA
"Walk softly and carry a big lens."
GEAR: 5DII, 7D(x2), 50D(x3), some other cameras, various lenses & accessories
FLICKR & PRINTROOM
