this is the predecessor to that 75mm howitzer mount, purpose built anti-tank version of the m3 (t-48 57mm/su-57):

over 980 built starting october 1942...supposedly none in u.s. service...630 sold to britain and russia...british kept 30 or so then sent the rest to russia...russian version used the more powerful 57mm zis-2 anti-tank gun very successfully...the germans even put captured ones to use.
the 75mm gun, motor carriage m3 was initially used in n.africa was considered obsolete by 1943 due to being ineffective against german armor....but it was used with great success throughout the war by the marines who faced mostly bunkers and thinly armored japanese tanks...

the m3 105mm howitzer mount was also produced, but mainly used as a mobile infantry support weapon not anti-tank operations...

if all 3 gun mounts were available in the hangar...it would be the most versatile gv in the setup...skins for the u.s., russia, britain and even germany could be created for it.