I'm very new to PHP, and I don't know that this is correct, but I believe this will work for you...
In Forumdisplay template, find:
Code:
<a href="newthread.php?$session[sessionurl]do=newthread&f=$foruminfo[forumid]">
And replace it with this:
Code:
<a href="<if condition="$foruminfo['forumid'] == xx">form.php<elseif condition="$foruminfo['forumid'] == yy">form2.php<elseif condition="$foruminfo['forumid'] == zz">form3.php<else />newthread.php?$session[sessionurl]do=newthread&f=$foruminfo[forumid]</if>">
xx is the forum ID for form.php, yy is the forum ID for form2.php, and zz is the forum ID for form3.php.
Back up your template first, but I believe this will work for you. If it doesn't, change the closing </if> tag to </endif>.