Glioblastome tissue segmentation using multimodal mri scans

Hi,
purpose is to extract the lesion using T1,T2 and flair images. I know the intensity values for the lesions also like if there are three different lesions than i have 1,2 and 4 intensity values. how i can extract these lesion using FSL commands.
I would appreciate your help.
Thank you.
Asma