If the Prisoner coin represents Angela, then could the other coins represent something too? The Old Man coin could represent her father and the Snake coin could represent her feelings of disgust after each time her dad raped her if you take the snake as meaning something phallic. What do you think?
I certainly agree that if the prisoner coin means something, the latter two must have some sort of symbolism too. The Old Man is most likely her father, while I believe the snake could contain phallic meaning like you said or could even allude to her mother. I'm not exactly sure why but in many cultures, snakes are personified as female, and if the other two represent humans, I find it most likely the last one will too. As we knows, when personified snakes are commonly considered sly, evil. This could easily match her mother. We may not know much about her, but it is given to us that Angela clearly wants her mother, and badly. But later in the game we hear her say "even Mama said I deserved it." Yet simultaneously, she's so attached to her... and this is quite associable to the stereotypical snake: Tricky but still gets what it wants, that being her daughter's support.
I don't find Ozzkat's idea very compelling or sensical, but I just bet you Deviantcorn is right. The idea of the coins (besides just the prisoner) meaning more hadn't occurred to me before, and the snake being phallic was the first thought I had too. Not enough so to add it to the article, as that's pretty much the definition of a fan theory, but I'll be damned if it's not a good one that I like.
You should also consider the places where the coins are found. The Coin of Old Man is found in garbage chute, and I first thought this could be a theme of abandonment of elderly, but in the context of the puzzle and narratives it gives in different difficulty modes it may play another role. Also the Coin of the Snake is found in very specific circumstances, which is in the stroller in an empty pool surrounded by lying figures, this whole arrangement instantly gave me the thought this by no way can be accidental, the main thing to take away is its probable connection with children and babies. Now connect the images from back sides of coins (gravestone for old man, crescent for snake, and nest full of eggs (which is akin in function to stroller and shares the same theme of fertility and childbirth, also maybe ki-no-ko connection?) for woman prisoner), and I'll live you at that. Also two of the coins feature greek text all capitals, and the prisoner coin has a different(?) text in lower case, on silent hill forum I found a topic concerning these, they came to the conclusion that this is translit from japanese and it ended there, apparently the japanese text should sound like "maremaseeta tatsukee ga kpsonen no funigatu". Doesn't make sense to me, not that I know much japanese, but I tried to translate this as まれましたーたつきがくそねんの不二月 and google translate casually threw "It was rare - Mr. Tatsuki fell a fuck of Fuji" at me. Well...The only connection with mentioned Mr. Tatsuki I found in Silen Hill is tatsuki pill that Maria takes in hospital. Anyway, if somebody knows japanese well enough, I think we will all appreciate insight into the secret of the coins.