How could i do that?
You can't. There is one "position" (the other Arduino) to send data to.
What you have to do is send sufficient data to the other Arduino so that it knows what to do with the data.
Your code, as posted, makes little sense, unless maybe you are familiar with the cap sense library.
CapacitiveSensor cs_7_2 = CapacitiveSensor(7,2);
CapacitiveSensor cs_7_3 = CapacitiveSensor(7,3);
CapacitiveSensor cs_7_4 = CapacitiveSensor(7,4);
You create three instances. Fine.
int sensor1 = cs_7_2.capacitiveSensor(30);
int sensor2 = cs_7_3.capacitiveSensor(30);
int sensor3 = cs_7_4.capacitiveSensor(30);
It is not at all obvious what this code is doing, or whether it makes sense to call these methods outside of a function.
int opcion=1;
if (opcion==1){
It doesn't make sense to test if the variable you just assigned a value to holds the value you just assigned.
int measure[]={sensor1,sensor2};
You have three sensors, so creating a two element array doesn't make sense.
sensor1 = cs_7_2.capacitiveSensor(30);
sensor2 = cs_7_3.capacitiveSensor(30);
if (measure[thisMeasure]>25) {
The fact that you, at some point in the past, copied the current value of sensor1 into the array does not mean that the array contains the value you just read.
It doesn't make sense to read all the sensors on each pass through the for loop when only one of the sensor's values will be dealt with.
The Wire.write() method does NOT take an int. The value in sensorN will be truncated to a byte, and sent to the other Arduino.
What you need to do is a create a 3 element byte array, and put the sensor NUMBER in the first position and the sensor reading's high and low bytes in the other two positions, and then use the write() method that takes an array and a count, to send the three element array.