I have always shot Apache (split finger) draw with a single layer cordovan tab (Alaska Bowhunter supply type)
Now I want to see about shooting 3under, Can I use the split finger tab to do this? any adverse effects by doing so? or do I need to buy a 3under tab?

basically the same tab w/o the split in it.

Others more experienced will chime in here but I don't think it matters. I have been shooting 3 under with my Cavalier Ultra Lite Elite tab and it is working just fine.

I'd say use the one you've got!

When I went to 3under, i initially used my split tab, but then bought a 3under black widow tab. Seems to work a little better for me and has a small tab above my index finger that goes against the arrow nock. The callous that I had developed there is finially going down some.

a split tab works fine

yup go for it no problem

Either will work fine. I do however use a BW 3 under tab!

When I went to 3 under, I started with a split-finger, single-layer cordovan tab and a glove.  I eventually picked up a few 3-under tabs (Black Widow cordovan, Bateman, custom hair-on cow or calf hide).  At the moment I prefer the split-finger, single-layer cordovan tab or a glove.  YMMV

I have used a split finger tab for three under in an emergency, but prefer a 3 under tab. We have wrestled with making the perfect tab and have spoken with lots of others to get their opinions. Talking with Lenny Rock resulted in our taking another approach to making them. He had one that he attributed part of his seccess to, and was generous enough to share with us. We have since made several versions of it using either calfskin or cordovan. I'll try to post a photo of a cordovan "Rocktab". It's not on the website yet but can be purchased by calling into the phone number given.

I can tell alot of difference in using a 3 under tab and a split tab for 3 under shooting.  It is in comfort not shooting.  I feel like I am more consistant with at tab that fits me and the style that I shoot.  Just me, IMHO.

Woodie,
       The only reason you should probably go with a tab specifically built for three under is that it will not allow you to take a "split" hold on the nocked arrow.We are creatures of habit,and when you have shot split for a long time and decide to go to three under sometimes you need a reminder until it sinks in your head. I made my own,single piece of leather,trace the outline of your split tab,cut it out less the split,make adjustments and you are good to go.
                                      Craig

Doesn't matter. I shot 3 under for a couple of years and used both types of tabs. I could see no difference.