Simple Variable/string Question

I have three drop down menus that when changed call a simple js
function, passing a value i.e.

<SELECT name="Team" onChange="change('national');">

the function defined at the beginning is

function change(which) {
current = document.premForm.Team.selectedIndex;
document.images.premShirt.src = "assets/shirts/"
+document.premForm.Team[current].value + ".gif";
}

the var 'which' is being passed succesfully, but i can't work out how
to change

document.premForm.Team.selectedIndex;

into

document.whichForm.Team.selectedIndex;

depending on what value the function receives. My 3 forms are called
premForm,nationalForm and worldForm