All of the chemicals in your problem are compounds. Only ionic compounds contain cations.
The definition of "cation" is a positively charged ion.
Here's how you can recognize a cation:
It is the metal part of an ionic compound. an ionic compounds is made up of a metal and and a nonmetal.
The metal is always written first in the formula.
Remember that metals are found on the left side of the step on the periodic table.
BF3: The first element, B, is not a metal. So this is NOT an ionic compound and there is no cation here.
BH3: Same thing as BF3
AlCl3: the first element, Al, is a metal. This is an ionic compound and Al is the cation (Al's charge here is +3).
By the way, Cl is the nonmetal negative ion here. It's called an "anion". (Each Cl has a charge of -1 here)
FeCl3: the first element, Fe, is a metal. This is an ionic compound and Fe is the cation (Fe's charge here is +3).
the left side is a cation
F- ; H- ; Cl- ; Cl -