Out of the three, Latvia has serious energy storage - a 4 billion cubic meter (at normal pressure) underground gas store, sufficient to carry all three countries over the winter.

But so far, it’s filled with fossil natural gas - but some day it could be filled with synthesized methane.

As a backup option, Estonia has oil shale - probably the worst fuel on Earth, so the price of emitting CO2 keeps those plants out of the energy market during summer. During winter, they come online though.

As for solar, we aren’t planning to rely much on that. Solar capacity has of course skyrocketed, but only because it’s very easy to install. But at latitudes 55 to 60, days are really very short in midwinter, so wind and waste wood are the likely candidates in future - after oil shale leaves the scene, but before synthetic gas becomes feasible.

Regarding pumped hydro - it can stabilize a day, but can’t stabilize a week or month. Lithuania has a biggish (~10 GWh) pumped storage facility. The rest of Baltics don’t have suitable terrain. Estonia has limestone banks, but they’re under various forms of protection and even if one built a lot of pumped hydro, the low elevation difference (up to 50 meters) means one couldn’t support the electric grid through more than a few days.

Regarding hydrogen - maybe.