You only need 1 hand crossbow (and it must be in your hand)
I assume you are referring to the the Crossbow Expert feat (since this is the only published feat that mentions crossbows). If you take this feat at level 4 and then gain Extra Attack at level 5, you can load and fire the same hand crossbow 3 times per turn: twice with your attack action and Extra Attack, and a third time with your bonus action using the Crossbow Expert feat. This is possible because the feat allows you to ignore the loading property, which would otherwise prevent you from using your Extra Attack with it.
In fact, I believe the only way to get the full 3 attacks per turn is to be holding exactly one hand crossbow in your hand. If you were holding 2 hand crossbows, the ammunition property would prevent you from reloading them after you fire them, because
you need a free hand to load a one-handed weapon
And if you could somehow wield a hand crossbow with your leg, it would no longer be considered a "one-handed" weapon, which would prevent you from using the bonus attack from Crossbow Expert. And you would still need a free hand to reload it anyway. So, given all of these complications, by far the simplest way to get the full value out of the Crossbow Expert feat is to wield a single hand crossbow in one hand while keeping the other hand free in order to load it, as described above.
You (probably) can't wield weapons with your legs
Certain weapons, like heavy crossbows, have the two-handed property, which says:
This weapon requires two hands when you attack with it.
This property clearly prevents you from wielding the weapon with your leg, or even with two legs: you need to use two hands. For all the other weapons without the two-handed property, I can't find anything in the rules that directly says they must be wielded in your hands. However, although there is no explicit "one-handed" weapon property, there is strong circumstantial evidence that the designers consider all non-two-handed weapons to be one-handed weapons. First, there is the Crossbow Expert feat, and the ammunition property, which both make reference to one-handed weapons. In addition, the rules for melee attacks say:
A melee attack typically uses a handheld weapon such as a sword, a warhammer, or an axe.
Similarly, the rules for improvised weapons also reference holding the weapon in your hands:
An improvised weapon includes any object you can wield in one or two hands, such as broken glass, a table leg, a frying pan, a wagon wheel, or a dead goblin.
Finally, the Sage Advice Compendium also references one-handed weapons in a few rulings, such as:
Can my character wield two heavy weapons if she has the Dual Wielder feat? No. The Dual Wielder feat lets you wield two one-handed weapons. Every heavy weapon in the Player’s Handbook also has the two-handed property.
This ruling strongly implies that any non-two-handed weapon is considered a one-handed weapon, or else it wouldn't be eligible for two-weapon fighting. In general, although it's never explicitly stated, the intent seems to be that having only two hands limits the number of weapons you can wield at once, with every weapon requiring either 1 or 2 hands to wield.