It depends on your DM, but I'd rule yes.

Sometimes, determining how much of the cost of a specific weapon or piece of armor (shields included) comes from its unique properties is very hard, so some DMs just say that the weapon can't be crafted in any other variant than the one presented (with the added rationale that if they wanted you to be able to craft different versions of it, the developers would have provided the cost for the specific effect in the usual weapon enhancements table/chapter).

Anyway (proving negatives is a hard thing to do, so be aware that I might be wrong) I've never seen any manual snippet preventing people from upgrading those weapons/armors/shields.

A +2 weapon costs 8000 gp on top of its being perfect (300 gp) and the price of the heavy mace (12). The mace of terror costs 30240 gp more.
Now, 38240 is not a multiple of 2000×(any squared number) so I'd rule this property costing a flat 30240 gp. While I don't think such a property (3 times per day only, with a low will save) is wort that much, I'm going to use this value in my calculations and determine that the +3 equivalent costs 10000 gp more, for a grand total of 48552 gp.

Improving the mace to the next level would be done, as it is usual for upgrading magic weapons, by asking someone who has the appropriate weapon enchanting feat and access to someone casting the required magic weapon spell every day for 100 days.

It depends on your DM, but I'd rule yes.

Sometimes, determining how much of the cost of a specific weapon or piece of armor (shields included) comes from its unique properties is very hard, so some DMs just say that the weapon can't be crafted in any other variant than the one presented (with the added rationale that if they wanted you to be able to craft different versions of it, the developers would have provided the cost for the specific effect in the usual weapon enhancements table/chapter).

Anyway (proving negatives is a hard thing to do, so be aware that I might be wrong) I've never seen any manual snippet preventing people from upgrading those weapons/armors/shields.

A +2 weapon costs 8000 gp on top of its being perfect (300 gp) and the price of the heavy mace (12). The mace of terror costs 30240 gp more.
Now, 38240 is not a multiple of 2000×(any squared number) so I'd rule this property costing a flat 30240 gp. While I don't think such a property (3 times per day only, with a low will save) is worth that much, I'm going to use this value in my calculations and determine that the +3 equivalent costs 10000 gp more, for a grand total of 48552 gp.

Improving the mace to the next level would be done, as it is usual for upgrading magic weapons, by asking someone who has the appropriate weapon enchanting feat and access to someone casting the required magic weapon spell every day for 100 days.