∵ Real numbers A, B and C that are not all zero become arithmetic progression, ∴ 2B = A+C.

Substitute b=(a+c)/2 into the straight line l: ax+by+c = 0 to get ax+y (a+c)/2+c=0.

It becomes (2x+y)+c(y+2)=0,

Because a and c are not all 0, and at the same time,

2x+y=0，

y+2=0，

X= 1，y=-2，

It is known that the moving straight line L passes through the fixed point Q( 1, -2),

Set points M(x, y), ∫pm⊥QM.

∴ Vector PM Vector QM=(x+ 1, y+2)? (x- 1，y+2)=x？ - 1+(y+2)？ =0,

Into an x? +(y+2)？ = 1,

So point M is on a circle with (0, -2) as the center and 1 as the radius.

Therefore, the maximum value of |MN| at point M(0, -3) is 2-(-3)=5.

So the answer is: 5.