To increase an amount by 3%, what
single multiplier would you use?

The right multiplier is 1.03. Because x*1.03=x*(1+.03)=x+.03x
And .03x is exactly  3% of x.

The multiplier would be 1.03. You are increasing the original amount, x, by 3% To increase a number by a certain percentage you take the percent it's being added times the number, and then add that to the original. So 100% of x + 3% of x = 1x + 0.03x = 1.03x. The original amount, x, is multiplied by 1.03, so 1.03 is the multiplier you would use.