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I don't believe 3rd-level relationships are supported, and even if you could hack it together filtering would be broken. Definitely accessibility from Velocity scripts is broken with even 2nd-level objects, which is a deal-breaker for me in many cases.
If the above is not achievable (Marketo lead <-- Custom object 1 <--- Custom Object 2 <--- Custom Object 3), our second thought was to implement custom fields in the Marketo person (standard object) that we'd have used otherwise in our Custom object 1 (user in our system).
That means our chain would be doable as Marketo Person <--- Accounts --> Campaigns.
Sure, that would work. (With the Velocity exception.)
Or you could think about using Velocity tokens themselves as your product catalog (this enables wider visibility).