Email Governor, prioritization of email sends

This is part of why engagement programs are so good. The order in which emails (or nested programs) are put into an engagement program will determine the order they get communications, on a set cadence. 

And, it can be hacked so there are never any casts without content. tech.gadventures.com/building-an-advanced-email-queue-with-marketos-engagement-programs-4999f01b5681.

Not quite what you are asking necessarily, but that functionality isn't really possible in Marketo - and when you think about it, it would be a very difficult thing to engineer as a feature. You'd need to create "hierarchies" of each email....but then what dictates what happens when two emails of the same hierarchy are due to be sent? It gets messy quickly. Engagement programs allow you to set this, so hopefully that works.

No there isn't a system wide function governing priority. The closest is email limits which prevents a record receiving any more emails if they've received X number of emails in a set period (e.g. 1/day or 2/week etc...)

However, the system does not automatically enqueue emails to be sent once the number of sent emails has fallen below the limit for the set period. It is up to you to resend single email blasts using filters "Not was sent email", alternatively, if the person missed an engagement nurture email, they'll have to wait until the next round of sends.

You can do this by maintaining a sequence by using "Campaign is requested" trigger and "Request Campaign" flow step. You need to setup the flow as one system (all 3 campaigns sending the email should talk to each other) - which sends the next emails in sequence, by requesting the desired "next" smart campaign.