how to understand "as ~ as ever"

Your analyses of (1) and (2) are both correct. They do indeed refer to the ‘as X as Y (is)’ construction.

Your analysis on (3), however, is not quite right. ‘Ever’ when used as an intensifier is confined (as far as I can think of) to three specific circumstances:

1. With comparatives: When used before a comparative adjective, ever intensifies the comparativeness of the adjective (not the meaning of the adjective itself) and means something like ‘increasingly’: “He had to borrow ever larger sums of money to cover his gambling debts”. [Registers: all]

2. With ‘so’: The phrase ‘ever so’ cannot really be split up or analysed, semantically: it just means ‘very’: “Oh, he was ever so bright as a child”. [Registers: informal, colloquial]

3. With interrogatives/indefinites: When used after an interrogative or indefinite pronoun/determiner (often cliticised), ‘ever’ intensifies the meaning of the interrogative/indefinite: “However are we going to do that?” – “I can see nothing whatever”. [Registers: mostly somewhat formal; not common in informal or colloquial use]

In other cases, ‘ever’ has a more literal meaning: ‘at any time’ (used in questions and negatives), or ‘at all times’ (used in positive statements, somewhat rarer than the negative/interrogative use).

This is also the case in the sentence you quote: it means ‘at any time in the past’ here, and it refers to the verb, ‘heard’. The placement of the adverb before the subject and verb is rather formal and somewhat archaic, but may still occasionally be seen even in Modern English. The verb is in the simple past tense in this example, which is also somewhat archaic—in current English, you would usually expect to see a pluperfect there. Thus, the sentence can be recast with no difference in meaning as:

[H]e stood Farrington a half-one, saying it was as smart a thing as he had ever heard.

The construction “(that) ever [subject] [verb]” is quite common in older literature and folk songs; for example, the traditional Scottish and Irish song The Parting Glass uses it several times (with ‘ever’ being contracted to its poetic/archaic variant ‘e’er’):

Of all the money e’er I had,
I spent it in good company.
And all the harm e’er I’ve done,
Alas! it was to none but me.

Oh, all the comrades e’er I had,
They’re sorry for my going away,
And all the sweethearts e’er I had,
They’d wish me one more day to stay.

These examples also show that the simple past tense and the perfect/pluperfect are fairly interchangeable in this particular construction.

You’re right. It means that he had never heard anything smarter.