I found the answers. In Riwayatu Hafs, both Ishmam and rawm may be done on the word ta2manna. In the case of rawm, 2 noons are pronounced, ta3manona, with only 1/3 of the zamma on the 1st noon being pronounced. The only books that I found which had this question in it said that these three letters were called Lithawiyyah, because their makhraj was close to the gums. Ther must be a better answer beacause, tip of the teeth is not very close to gums at all