Calculation from user input

I work at a casting company & sometimes have to calculate weights. I am building a simple app in android studio to do this & was hoping someone could help as im struggling. I have 3 user inputs set, but I want to take those inputs & run them through a calculation & the have the result displayed at the end. I want to set a button so when I have the 3 imputs & can click go, & the result gets displayed. This is my code at present.

Code:

---

package com.example.nealu.myapplication;

import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.view.Menu;
import android.view.MenuItem;
import android.view.View;
import android.widget.EditText;
import android.widget.TextView;


public class MainActivity extends AppCompatActivity {


    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
    }

    public void onButtonClick(View V) {
        EditText e1 = (EditText)findViewById(R.id.editText);
        EditText e2 = (EditText)findViewById(R.id.editText2);
        EditText e3 = (EditText)findViewById(R.id.editText3);
        TextView t1 = (TextView) findViewById(R.id.textView);

    }
    @Override
    public boolean onCreateOptionsMenu(Menu menu) {
        // Inflate the menu; this adds items to the action bar if it is present.
        getMenuInflater().inflate(R.menu.menu_main, menu);
        return true;
    }

    @Override
    public boolean onOptionsItemSelected(MenuItem item) {
        // Handle action bar item clicks here. The action bar will
        // automatically handle clicks on the Home/Up button, so long
        // as you specify a parent activity in AndroidManifest.xml.
        int id = item.getItemId();

        //noinspection SimplifiableIfStatement
        if (id == R.id.action_settings) {
            return true;
        }

        return super.onOptionsItemSelected(item);
    }
}

---

I think im going wrong on the edit text bit but I have searched around & tried taking bits out of code for a calculator, but I need to do a little more than 10*5 etc when working a weight of a casting out so need to take the input & run it through a formula.

Any help would be greatly appreciated.

Thanks.