I think I have a plan. Now, let's execute this step by step: identify each word, check if it's a proper noun, replace it with three synonyms if not, ensuring proper formatting. Need to be careful with edge cases and ensure accuracy where possible.

I also need to handle different parts of speech. For example, "run" can be a verb or a noun. Depending on the context, the synonyms might differ. So I have to consider the part of speech when choosing synonyms.

Another consideration: contractions like "don't" or "they're" should probably be split into their components. But the user didn't mention contractions, so maybe leave them. Words like "don't" could be do not but not sure if that's correct. Maybe better to avoid contractions or handle them as part of the sentence.

Wait, the user said "all words with 3 synonyms," but articles like "the" are hard. Maybe they expect to leave some as is if there aren't three, but the user's instruction says to convert all words with three synonyms. Hmm, maybe the user made a mistake but I have to follow the instruction. So for words like "the," perhaps find three even if they're not perfect.

Let me test a sample input. Suppose the input is "Alice walked quickly through the park." Proper nouns here is "Alice," so we leave it. "walked" could be strolled. "quickly" as fast. "through" as via. "the" as a maybe? Though "the" is an article and might not have three useful synonyms. Maybe a as placeholders? The user might expect that.