As Goosegg says, the standard is "ordinary effort."
You have to make a subjective decision. Considering the totality of circumstances--where the fielder was positioned, where and how the ball was hit, and other factors in play like wind and sun--should a player of average ability at that level of baseball be able to make the play while exerting ordinary effort?
One unofficial rule of thumb I use is to put myself in the position of the pitcher. Could the pitcher have reasonably expected the play to be made by average players at that level?
Situation #1: Converging fielders can be a tough play at all levels. It's hard to look up into the sky, run at full speed and maintain awareness of others doing the same thing. Without seeing the play, it's easy to envision situations in which it isn't an error.
Situation #2: Sounds like the fielder had plenty of time. You need to decide if an average fielder at that level should be expected either to wear sunglasses or know how to use his glove to shield the sun. If yes, it's an error.
Situation #3: You decide if the weather turned an ordinary effort play into something harder than that.