What does it mean when you call 'convert to int'? How should it look like in the output? Your first 3 bytes already stored as int values, but its representation is still ascii characters.
If it meant to be string like "175125Bram" then solution should be like this
#include <sstream>
stringstream ss;
ss << buffer[0] << buffer[1] << buffer[2];
receiveStr += ss.str();