Let's rewrite the conditions in form of a system: there exist natural numbers k,m,n, for which equalities are true:
2a−1=kb,2b−1=nc and 2c−1=ma.
It is obvious, that all the numbers k,m,n and a,b,c are odd.
Then we have, that
b=k2a−1⇒2b−1=k4a−2−1=nc⇒c=kn4a−2−k⇒2c−1=kn8a−4−2k−1=ma⇒8a−4−2k−kn=knma⇒a=8−knm4+2k+kn.
Now we see, that an equality must be true: knm<8. Then from symmetry of conditions and oddness of numbers k,m,n it follows, that the following variants are possible (with accuracy to cycle).
Variant 1: k=m=n=1. Then we have c=2b−1⇒a=2c−1=4b−3⇒b=2a−1=8b−7⇒a=b=c=1, but numbers a,b,c must be different.
Variant 2: k=3,m=n=1. Then we have c=2b−1⇒a=2c−1=4b−3⇒3b=2a−1=8b−7⇒5b=7 – contradiction, because b is not integer.
Variant 3: k=5,m=n=1. Then we have c=2b−1⇒a=2c−1=4b−3⇒5b=2a−1=8b−7⇒3b=7 – contradiction, because b is not integer.
Variant 4: k=7,m=n=1. Then we have c=2b−1⇒a=2c−1=4b−3⇒7b=2a−1=8b−7⇒b=7⇒c=13,a=25 and this set satisfies the condition.