There's no 3-day MTR pass without the AE portion. However, there's a 24-hour MTR pass for HK$50. Hardly cheap, and few people use it.

Octopus is simply a cash-card system. You can get a regular Octopus for all your transportation and convenient store/supermarket needs. $150 to start card with $100 of value. You'll get ~$43 back the day you depart. It's not a pass, and each fare (bus, ferry, MTR, etc...) will be deducted from your credit.