I think you can create different tabStyle for each tab.
It would be something like this:

TabBar
{
firstButtonStyleName: “tabStyle1″;
tabStyleName: “tabStyle2″;
lastButtonStyleName: “tabStyle3″;
}

.tabStyle1 {
disabledSkin: Embed(source=”flex_skins.swf”, symbol=”Tab_disabledSkin”);
downSkin: Embed(source=”flex_skins.swf”, symbol=”Tab_downSkin”);
overSkin: Embed(source=”flex_skins.swf”, symbol=”Tab_overSkin”);
selectedDisabledSkin: Embed(source=”flex_skins.swf”, symbol=”TabSelected_disabledSkin”);
selectedDownSkin: Embed(source=”flex_skins.swf”, symbol=”TabSelected_upSkin”);
selectedOverSkin: Embed(source=”flex_skins.swf”, symbol=”TabSelected_upSkin”);
selectedUpSkin: Embed(source=”flex_skins.swf”, symbol=”TabSelected_upSkin”);
upSkin: Embed(source=”flex_skins.swf”, symbol=”Tab_upSkin”);

}

.tabStyle2 {

}

.tabStyle3 {
}

I found useful info about skinning here:
http://www.adobe.com/devnet/flex/articles/flex_skins.html