variants aren't texture based, they are model based so you would need to use custom models for this and have some modelling experience.
to do so you need to add a component to the model called extra1, extra2 and extra3 under the chassis dummy containing the variations... I guess in this case you may have to duplicate the shamal exterior in each extra though I'm not a modeller.
However if you just want to add 3 textures then you may be able to use per vehicle shaders in combination with a custom model so you have a texture to replace:
edit the model so it has a texture say "textureExterior" replace it with custom models then use shaders to edit that texture on a per vehicle basis
(since you only need 3 images you can create 3 shaders then apply them to each vehicle on creation)