You just need to divide by the overall sum -- that will normalize the values to a total of 1. Start with the sum of all 3, and then divide each one by the sum to get the normalized probability.
double[] probabilities = { 1, 1, 1 }
double sum = probabilities.Sum();
double[] normalized = probabilities.Select(prob => prob/sum).ToArray();
Or, working with your example:
void GetRanges()
{
double sum = clutteredObjects.Sum(obj => obj.spawnChance);
for (int i = 0; i < clutterObjects.Length; i++)
{
clutterObjects[i].spawnChance /= sum;
}
Spawn();
}