Try:
IF({number of trucks} = "3 trucks of more", "0") &
IF({number of trucks} = "1 truck (24 tons)", "24") &
IF({number of trucks} = "2 trucks (48 tons)", "48")
You could also use a SWITCH() instead:
SWITCH(
{number of trucks},
"3 trucks of more", "0",
"1 truck (24 tons)", "24",
"2 trucks (48 tons)", "48"
)