UnitConvert[Quantity[3.56, "USD"], "CAD"]/UnitConvert[Quantity[1, "Gallon"], "Liter"]
or
WolframAlpha["$ 3.56/Gallon in CAD/Liter"]
or just use the content from the last command to ask Wolfram Alpha online (instead of "CAD" you may also write "Canadian cents" to have the result in cents).