Comparing Multiple machine VS single machine requirement

Hello !
In our project we have recommended our customer to use 3 different macnines
while deploying the final system. Different module will be runing on each
machine to give the required performance .

Now the customer is insisting , instead of 3 machine ,take a single machine
which is 3 to 4 time higher configuration(multi core CPU, higher RAM, Higher
HDD, more communication ports etc) and run all the modules on the same
machine.

Q1 .How to decide(what parameters), whether this proposal is acceptable or
not ?
Q2. Is it theoritically right to expect the same performance with the higer
configuration PC ?
Q3. What kind of bottelneck we will face ,if we adopt this proposal ?

Regards
Manish