Well, for the Golden Age....hmmm, difficult for the final member, but the first two are obviously:

Alan Scott/Green Lantern
Jay Garrick/The Flash

The third could have been either Dr. Fate (Kent), or Hawkman (Carter), but I went with Carter.

A modern era JSA is also not easy.

Kara Zor-L/Karen Starr/Power Girl would be my first choice. And leader. She's the first of the JSA's second generation, or at least the most important of that era.

Holt's Mr. Terrific would be my second. A newer, second gen version of the original.

Stargirl would be my third, representing the youngest and newest part of the JSA.