That took a while, but I understood it better upon reread—the one in relationship with Short was Curly, and they were mutually in love.
So, long story short (hehe), Long liked Ponytail, and Ponytail liked Short, but Short was dating Curly.
Long wanted to keep Ponytail "happy", but at the same time she didn't want to lose Ponytail, so she devised a plan:
- Make Short break up with Curly. She did this by seducing Short and blackmailing her.
- Date Short after her breakup and boast about it to Ponytail, so Ponytail would never confess to Short.
Short was, obviously, unhappy about this arrangement, so she kept dating Curly in secret while keeping her arrangement with Long under wraps.
The rest went down as in the manga—Long found out that Short was still with Curly, Curly had no idea what was going on and ended up distrusting Short, Ponytail dumped Long for hurting everyone, and Short was left with nobody on her side.
Ponytail won in the end.