I was expecting you to say a much larger distance, to be honest. Three hours isn't so bad, especially given how cheap you can get tickets if you book in advance. I used to date a man in Liverpool: it was 4-5hrs by train and we saw each other every other month. That was all we could afford—we earned approx £1400 per month between us—but if we'd had the money it would have been more frequent. Even with the pandemic and her mother's diagnosis (I'm sorry to hear about that), it sounds like she could be putting a lot more effort into your relationship. If you want to be with someone you should be making the effort to make it work - or at least that's what I believe, anyway.
My girlfriend and I are also at risk of having to do long-distance very soon, as I may need to move to the Highlands for two years. It would be about 3hrs by train. We've already set an agreement that we spend at least one weekend together a month; two if we can manage it (and we've also talked about her potentially moving with me). The thought of only seeing her once a month already terrifies me, given we spend 4-5 nights a week together right now. I couldn't imagine not seeing her for 2+ years. I'd be an insecure wreck if we had to physically be apart for that length of time and I know the feeling is mutual.
Waffled a bit there, sorry, but I genuinely don't know how you cope with it. I know I'd be unhappy if I were in your shoes. It might be a good time to sit down and have a conversation about where the two of you are currently at in this relationship, what both of your individual needs are, and how you can work together to meet those?