In a survey of 120 people, the number of users for WhatsApp (W), Instagram (I), and TikTok (T) are given. Calculate the number of people using only one app and exactly two apps.

Only WhatsApp: $|W \text{ only}| = |W| - (|W \cap I| + |W \cap T|) + |W \cap I \cap T|$

Only Instagram: $|I \text{ only}| = |I| - (|W \cap I| + |I \cap T|) + |W \cap I \cap T|$

Only TikTok: $|T \text{ only}| = |T| - (|W \cap T| + |I \cap T|) + |W \cap I \cap T|$

The number of people who use only one app is $\boxed{\text{60}}$.

2. How many use exactly two apps?
This is the sum of people who use (WhatsApp and Instagram only), (WhatsApp and TikTok only), and (Instagram and TikTok only).

WhatsApp and Instagram only: $|W \cap I| - |W \cap I \cap T| = 35 - 15 = 20$
WhatsApp and TikTok only: $|W \cap T| - |W \cap I \cap T| = 28 - 15 = 13$
Instagram and TikTok only: $|I \cap T| - |W \cap I \cap T| = 22 - 15 = 7$

The number of people who use exactly two apps is $\boxed{\text{40}}$.

3. How many use none?
First, we find the number of people who use at least one app using the Principle of Inclusion-Exclusion.

The number of people who use none is $\boxed{\text{5}}$.