Operator that combines sum, product and difference between two numbers
JavaScript (ES7), 63 61 59 bytes

Saved 4 bytes thanks to Neil.

(a,b)=&gt;[a*b,a+b,a-b].reduce((p,c)=&gt;p*10**-~Math.log10(c)+c)




&lt;input id=a type=number oninput="c.innerText=((a,b)=&gt;[a*b,a+b,a-b].reduce((p,c)=&gt;p*10**-~Math.log10(c)+c))(+a.value,+b.value)"&gt;
&lt;input id=b type=number oninput="c.innerText=((a,b)=&gt;[a*b,a+b,a-b].reduce((p,c)=&gt;p*10**-!Math.log10(c)+c))(+a.value,+b.value)"&gt;
&lt;p id=c&gt;