The problem with the code is, when I try to generate a number, if the spin is equal 1 it generates values inside range (1,2,3) if if try to use the loop to sum random values
You need to initialize your Random object, then call Next() inside your loop.
i.e.
if (toggled3.Checked)
{
// initialize your total and the random number generator
int diceTotal = 0;
Random rand = new Random();
for (int i = 0; i < spind3.Value; i++)
{
// add the next random number between 1 and 3
diceTotal += rand.Next(1, 4);
}
}