Call good a number n∈N for which there exist a,b,c∈N≥1 such that
n=(a,b)⋅(b,c)+(b,c)⋅(c,a)+(c,a)⋅(a,b).
We will prove that, regardless of the value of M, the good numbers are those of the form n=22t(2k+1), where t,k∈N, k≥1.
Let k≥1 and p,q,r>M three distinct primes such that r>max{kp,kq}. Considering a=kp,b=kq and c=r, we have (a,b)=k and (b,c)=(c,a)=1, which leads to (a,b)⋅(b,c)+(b,c)⋅(c,a)+(c,a)⋅(a,b)=2k+1, so 2k+1 is good.
Let t∈N and n be a natural number for which there exist a,b,c>M, pairwise distinct, such that n=(a,b)⋅(b,c)+(b,c)⋅(c,a)+(c,a)⋅(a,b). For every t∈N, we have 2ta,2tb,2tc>M and 22t⋅n=(2ta,2tb)⋅(2tb,2tc)+(2tb,2tc)⋅(2tc,2ta)+(2tc,2ta)⋅(2ta,2tb), so all the numbers of the form n=22t(2k+1) are good.
Further, we prove that the numbers of the form n=2t or n=22t+1(2k+1), where t,k∈N, k≥1, are not good, so they don't satisfy the conditions of the problem either.
First, we prove that any good number that is even is actually divisible by 4.
Consider n0 an even number. If n0=(a,b)⋅(b,c)+(b,c)⋅(c,a)+(c,a)⋅(a,b), then (a,b), (b,c) and (c,a) can't be simultaneously odd. For example, if 2∣(a,b), then 2∣(b,c)⋅(c,a), so a,b and c are even (the other cases are completely similar). It follows that 4∣n0. Also, the number 4n0 is good too, because 4n0=(a′,b′)⋅(b′,c′)+(b′,c′)⋅(c′,a′)+(c′,a′)⋅(a′,b′), where a′=2a,b′=2b,c′=2c.
Suppose, for the sake of contradiction, that there exists t≥2 such that n=2t is good. Consequently, 2t−2,2t−4,2t−6,… etc. are also good, so either 1 or 2 should be also good. This is a contradiction, since if a,b,c≥1, then (a,b)⋅(b,c)+(b,c)⋅(c,a)+(c,a)⋅(a,b)≥3.
Similarly, if n=22t+1(2k+1), with t,k∈N, k≥1 is good, then 22t−1(2k+1), 22t−3(2k+1),…,2(2k+1) are also good, impossible, since 2(2k+1) is not divisible by 4.