There’s been a similar question just 10 days ago:

Krakow or Warsaw - Vilnius - Riga - Tallin | Community (eurail.com)

It’s possible to use your pass within these countries but it’s not worth using a travel day in the baltic countries (of course that doesn’t matter if you are a holder of a continous pass). 

It is possible to travel there but:

• Estonian and Latvian railways haven't sent their timetable to the European Timetable Centre, so you can only check them on their own websites and you'll have to enter the trains manually into the app. Lithuanian timetables are in the app though.
• Normal tickets are quite cheap so if you have a flexipass, check if it's worth using a travel day for.
• There's one train/day from Poland to Lithuania, no trains from Lithuania to Latvia and only a one train/dag with often a long wait between Latvia and Estonia.

If you are only planning to travel to the Baltic states have you considered using just buses? You might want to compare prices on a rail pass (and, as said, rail in these states is lacking connection) and via buses. I don’t know if I am allowed to mention the bus operator here.

I myself am planning to travel the Baltic states this year. I plan to fly from UK to Helsinki, cross the water to Estonia and then travel by bus between each capital. I will then start my Interrail ticket in Vilnius and travel to Bialystok.

Talinn to Riga, for example, is 14€ by bus.

https://www.interrail.eu/en/plan-your-trip/tips-and-tricks/trains-europe/trains-country/trains-lithuania

Those country pages are very handy but are a bit hidden on the website.

If you’re travelling on past Estonia to Finland then I believe you can get discount on buses with Interrail, though I did read the offer needed updating.

I really did consider using trains in the Baltic States though. Think having to find conenctions would be a bit of an experience. But I get an extra week using cheap buses. This video is interesting