3 isomers

##C_3H_8O## has three constitutional isomers:

1-propanol: ##CH_3-CH_2-CH_2-OH##

2-propanol: ##CH_3-CH(OH)-CH_3##

ethyl-methyl ether: ##CH_3-CH_2-O-CH_3##

There are no geometric isomers or stereoisomers posssible for these structures