Put some grease on the splines and slide it into the third member all the way you get it to go and then remove it to see how much it went into the side gears and into the rest of the parts
I've broken, twisted off, more than one stock left side A body axles and they all went way more into the rear end ( 7/8 to 9/10 of the splines) than the one John showed in his picture
Yours should go that far in also