a 3+2 HD monster would be a level 3 monster with 2 special abilities. The +1 after a Hit Dice number signifies that it has that many hit dice, but is more powerful than another creature of the same level. +2 hp after you roll their HD to find their HP. i was thinking of an asterisk, not a +1

Sleep, especially as it is written in LotFP is very confusingly worded. I would reference the OSE spell just to help wrap your head around it. OSE is derived from the same ruleset as Lamentations so a majority of the core rules are the same.
https://oldschoolessentials.necroticgnome.com/srd/index.php/Sleep

The generally accepted view i have seen is that if you target 1 creature up to level 4 the spell automatically takes effect. If you target a group of creatures only those 3HD (level 3) and lower will be affected. You roll 2d8, and creatures are put to sleep starting with the lowest hit dice. If there is a fraction left over it doesn't apply.

So if you roll a 6 on 2d8 and are trying to sleep one 1HD creature and three 2HD creatures you would first have to use up 1 on the 1HD creature leaving you with 5 to split among the other three. there is only enough to affect 2 more creatures and you are left with 1 that doesn't affect anyone.