Say, can I put 3 advancement tokens on Hostile Takeover?
Yes, you can. From the rulebook, p. 14
Agendas can always be advanced while installed. Cards other than agendas can only be advanced if their text box allows it. There is no limit to the number of times a card can be advanced.
Scoring Agendas– When the number of advancement tokens on an agenda is equal to or higher than its advancement requirement, the agenda is fully advanced and the Corporation can score it
Absolutely. The rules state that "There is no limit to the number of times a card can be advanced." and you can score an agenda if "the number of advancement tokens on an agenda is equal to or higher than its advancement requirement." In fact this is actually intended for some of the agendas.
Take a look at Project Beale, which is the most notable example. Advancing past the point you need to on Beale gives more points. Braintrust works the same way, but lowers the rez cost of ice rather than being worth more points. Project Ares makes the runner trash cards when it is scored, one installed card for each advancement over it's requirement.
Project Atlas, Project Kusanagi and Project Vitruvius all also gain agenda counters for each advancement over the required when you score them, but these unlike Beale and Braintrust are spent to activate an effect. Overadvancing more gives you more uses of searching R&D for a card, adding subroutines to ice, or returning cards from archives to HQ.
Another example group are the public agendas - Underway Renovation, Oaktown Renovation, Hollywood Renovation and New Construction. Each of these agendas are installed face up, and give a bonus when you advance them (milling the runner, gaining credits, advancing other cards and installing cards) these bonuses get better once the agenda can be scored but is still being advanced (more cards, more credits, more counters, rez the card free)