Your solution looks alright. I think you could use a defaultdict() to clean up a few lines of code, and I don't fully understand why you have two nested loops inside your file processing loop.

Here's my solution in TS.

    const parseLog = (input: string) => {
        const userToHistory: {[user: string]: string[] } = {}
        const pageListToFrequencyCount: { [pages: string]: number } = {}

        for (const [user, page, ] of input.trim().split("\n").map(row => row.split(", "))) {
            userToHistory[user] = (userToHistory[user] ?? []).concat(page);

            if (userToHistory[user].length >= 3) {
                const path = userToHistory[user].slice(-3).join(" -> ")

                pageListToFrequencyCount[path] = (pageListToFrequencyCount[path] ?? 0) + 1;
            }
        }

        return Object.entries(pageListToFrequencyCount).sort(([, a], [, b]) => a - b);
    }

It could be slow on large log files because it keeps the whole log in memory. You could speed it up significantly by doing a `.shift()` at the point when you `.slice(-3)` so that you only track the last 3 pages for any user.