I have all 3 cards, because some places don't accept AMEX.
Denise I was going to put a formula together for you but it is really as simple as how much do you want to pay in annual fees versus the benefits each card provides. The following goes with the assumption you stated that your primary goal is to accumulate miles.
Both the Hawaiian and Alaska cards provide 3 miles for each $1 charged for their respective airfares. Everything else is 1 mile per $1.
The SPG Amex (post August 1) will provide 2 miles for each $1 charged for Vistana MFs and Starwood/Marriott hotel charges. (I am trying to get the Vistana MFs verified in writing.) For everything else the SPG Amex will only provide 2/3 of a mile per $1 charged. Therefore, this is not the card to charge anything to except those charges listed above.
The three cards have a total of $269 of annual fees. Alaska - $75; Amex - $95; Hawaiian - $99
If you have several thousands of dollars of Vistana MFs and charge as much as possible to your villa when staying at a Vistana property, then it is probably worth keeping the SPG Amex. This is especially true considering the 5,000 mile bonus (25%) when transferring 20,000 Starpoints (60,000 Marriott points in the future). Additionally, if you don't have elite status with Starwood/Marriott the card will give you Silver status. Silver status doesn't have many great benefits but it does provide a 10% bonus on points awarded for stays at Starwood/Marriott hotels. Also going forward this card will provide annually one free night in up to a Category 5 property.
As to the two airline cards, both out earn the SPG Amex (even considering the 25% bonus) for charges of Alaska and Hawaiian airfare. Additionally, if you do not have elite status with either airline, the first bag free baggage limit on both of those cards can easily pay for the annual fee with just a couple of roundtrip flights. Those cards also provide an annual companion fare/discount.
If it were me and my number one goal is the accumulation of miles, then I would keep all three cards assuming I had enough Vistana charges/MFs to make the SPG Amex worth having.
That's my two cents worth.