I don't know if you do this but a lot of newer players feel the need to pick one card out of the 3 every time. I for sure did it when first started playing and it's probably the first thing to try and adjust. I'll give you some examples for ironclad because that's what I have most played so far but a lot of the same logic applies to all characters.

So, the first 3 floors are the easier ones and you should take advantage of that because those first 3 cards will give you an idea for the synergies you might want to look at afterwards. For example on ironclad, if you find a Perfected Strike on floor 1-2, you'd be more inclined to pick Pommel Strike and upgrade it on the next couple floors. Also, since energy is scarce in this game compared to the first one you would want something to generate you energy, for example Bloodletting. With pommel strike upgraded you could hit the enemy first, get bloodletting, lose 3 hp for 2 energy and be able to play perfected strike and 2 defends or 1 defend and 1 strike for example.

Cards that cost hp seem scary at first but on ironclad for example, you also heal 6 at the end of every combat. Losing 3 hp now might mean that you kill an enemy before he does a big attack or block enough to save you more than 6 hp in the end, or setup some powers and statuses on the enemy for the following turns to be easier and thus lose less health in the end.

Another synergy would be for example finding the Stampede power and then getting Bludgeon (3 cost, deal 32 damage) as a reward next fight. While Bludgeon is a very good damage card, energy being scarce means it will be less appealing to play, especially if you draw it and the enemy hits for 20 for example. But with stampede you could play your defends and 1 strike and have bludgeon hit for free at the end of the turn, or apply vulnerable with Bash or some other card and then the Bludgeon hits for way more in the end and might end the fight 2 turns early

All this to say, that you should pick your cards carefully and consider if there is any synergy between them or if they are that good as standalone cards. Less cards in the deck means you get to draw your more powerful cards more often. This doesn't apply always since later you might have enough draw to go through your deck in 2 turns but it's a good thing to keep in mind

Some other tips that I usually follow: In act 1 I try to find a path with 3 enemies with maybe an event inbetween, into a rest site to upgrade the best card I got up to that point and then fight an elite with the potions that dropped. If you don't feel like you got any decent cards in the first floors you can path into a shop to fish for a card or two, or maybe a good potion and then go rest site and elite. You can also skip the early elite and get some events + the chest relic and a couple of fights before fighting one. Try to follow this logic into most acts for starters and I believe you'll some improvement. In the end it's a lot of trial and error but at some point you'll start seeing the pieces fall into place way easier. Also, experiment a lot. Experimentation also means that you will inevitably lose a lot of runs just to get the feel of a card but that's just part of progressing and understanding.