Placeholder for unknown number of digits

I would like to add placeholder on a HTML input type which could hold number between 1 and n (unknown large number of digits, for practical reasons it will be limited to some number, let say 10). This input type is a part of a "input group" (three fields which combined hold an information) which contains another two input types, first one having fixed size of two allowed digits and second one of three allowed digits. I was thinking to use hash ('#') sign as a placeholder for first two fields, however, I am not certain how to deal with third input.

<input type="text" placeholder="##" /> 
<input type="text" placeholder="###" />
<input type="text" placeholder="?????" />