“They want exclusivity after three dates—is that too soon?” OR “How long should you date before being exclusive?”
"Too soon" is a silly notion, in my opinion. The real question is, do you want to be exclusive? Do you see yourself wanting to be with other people? Do you enjoy your time together enough to focus your efforts only on them?
You can decide to be exclusive with someone without having gone on a single date. You can decide to be exclusive with someone after ten dates. If your partner wants to be exclusive, but you don't - it is a kindness to tell them and let them go.
The right time to become exclusive is whenever you and your partner feel ready too be exclusive.
“Who should initiate the exclusivity conversation?”
You, probably. If you want to be exclusive, bring it up! It is a funny dance when you are both tiptoeing around the conversation. Chances are, they are nervous to bring it up, too.
“What signs suggest they want to be exclusive?”
If a person hints that they aren't seeing other people, or they mention they have deleted the apps, that is a good sign they are moving toward becoming exclusive.
“How is exclusivity different from a committed relationship?”
I think this is a really interesting question because it underscores how loaded the word "committed" is. The reality is, exclusivity and commitment are often tied together, but have different time frames on them. Commitment is a long-term oriented relationship frame - I am dedicated to this relationship lasting. Exclusivity is a more close-term oriented frame - I won't pursue a relationship with anyone else while we are together. You can be exclusive without being committed, and you can be committed without being exclusive, but what you are should be clear between both partners.
“What are the expectations when dating exclusively?”
There are certain social norms around exclusivity, but as with many things in relationships, the best approach to deciding expectations is to have an open conversation about it. The most common assumptions people make are going to be around romantic and sexual exclusivity. Most commonly, being exclusive means you wont pursue a romantic OR sexual relationship with anyone outside of your partner. If you aren't sure what your partner expects from being exclusive, ask them about it.