Use Modloader or IMG Manager 2.0 to import files from the folder "00 - Import to gta3.img" to archive [GAME FOLDER]\models\gta3.img:
dumper.dff
dumper.txd

Changes in text files

In file [Game folder]\data\vehicles.ide
Find the line that starts with
406, 	dumper,		dumper,	 	mtruck, 	DUMPER,		DUMPER,	
Replace found line with
406, 	dumper,		dumper,	 	mtruck, 	DUMPER,		DUMPER,	truck,	worker,	 	5,	1,	0,		-1, 2.28, 2.28,		-1

In file [Game folder]\data\handling.cfg
Find the line that starts with
DUMPER 
Replace found line with
DUMPER 20000.0  200000.0   4.0    0.0 0.5 -0.4  90  0.78 0.8  0.55  	4 110.0 25.0 30.0 R D 	03.17 0.40 0 30.0  	0.8  0.06  0.0   0.20 -0.30 0.55 0.0		0.45 0.20 5000  	c008		1340201		0  1	20

In file [Game folder]\data\carcols.dat
Find the line that starts with
dumper, 
Replace found line with
dumper, 1,1