Transfer function

A question has given me 3 graphs of a step responce of a real life system and asked me to work out the transfer function

I've been given vo(s)/vi(s) =[k(e^-tds)]/ts+1

Where k is the ratio of output and input
td is delay
t is time constant

Do I just work out the values from the graph and substitute the values into the equation they gave me?

So I would have 3 transfer functions

In addition the unit step had an amplitude of 3 would this effect the transfer function or when I want to work out the step response I would need to multiply the transfer function by 3/s rather than using 1/s then convert to time domain?