From NutritionData. All nuts are raw and unsalted.

Almonds
One ounce, 23 kernels
Calories: 162
Carbohydrate: 6.1 grams (3.4 grams fiber)
Fat: 14 grams (1.1 sat, 8.7 mono, 3.4 poly) 1.7 mg omega-3, 3408 mg omega-6, 1:2005 ratio
Protein: 6 grams
Calcium: 74.6 mg
Magnesium: 75.7 mg
Vitamin E: 7.4 mg

Walnuts
One ounce, 14 halves
Calories: 185
Carbohydrate: 3.9 grams (1.9 grams fiber)
Fat: 18.4 grams (1.7 sat, 2.5 mono, 13.3 poly) 2565 mg omega-3, 10761 mg omega-6, 1:4 ratio
Protein: 4.3 grams
Calcium: 27.7 mg
Magnesium: 44.6 mg
Vitamin E: 0.2 mg

Pistachios
Once ounce, 49 kernels
Calories: 157
Carbohydrate: 7.9 grams (2.9 grams fiber)
Fat: 12.6 grams (1.5 sat, 6.6 mono, 3.8 poly) 71.8 mg omega-3, 3729 mg omega-6, 1:52 ratio
Protein: 5.8 grams
Calcium: 30.2 mg
Magnesium: 34.2 mg
Vitamin E: 0.6 mg

Pumpkin Seeds
Once ounce, 142 seeds, bit less than 1/4 cup
Calories: 153
Carbohydrate: 5.0 grams (1.1 grams fiber)
Fat: 13.0 grams (2.5 sat, 4.0 mono, 5.9 poly) 51.1 mg omega-3, 5849 mg omega-6, 1:114 ratio
Protein: 6.9 grams
Calcium: 12.1 mg
Magnesium: 151 mg
Vitamin E: 0 mg
Iron: 4.2 mg
Zinc: 2.1 mg

What I learned:

1. Walnuts are a good source of omega-3. They also have a pretty good omega-3/omega-6 ratio of 1:4.
2. Calories are similar among the three.
3. Walnuts are the fattiest, most of it coming from polyunsaturated.
4. Pistachios and almonds – especially almonds – get most of their fat from monounsaturated.
5. Almonds are a great source of vitamin E, about half the RDA in one ounce!
6. Almonds are a respectable source of calcium.
7. Who doesn’t like pistachios?

I added the pumpkin seeds later. Similar in calories to the others. Very high in omega-6 relative to omega-3. A good source of iron and zinc (2 minerals vegans could use).

Keep in mind that the calories and fat listed for these nuts are not the calories and fat we absorb. Listed calories can be overestimated by up to 30% depending on how well we chew them, i.e. we get more fat and calories from almond butter than from the same amount of whole almonds: We Absorb Fewer Calories When We Eat Whole Foods (The Case Of Almonds)