Double thresholding for Region Growing

Hi. Can anyone please tell me how to perform Double thresholding for Region Growing without using the bwmorph function? I have found the two thresholds (t1,t2) using multithresh function, and hence I know there should be 3 regions. I also got a binary image for (t>=t1 & t<t2). I do not understand how to segment the histogram to grow the regions (say R1,R2 and R3) to finally get a binary image with only two regions. Thanks.