These ones (SM-3s) cannot... yet. A MIRV'd interceptor is under development (for both the SM-3 and the larger GMD system that is US based).
The white things are the actual interceptors, the gold thing is the core they're launched attached to. This is the one meant for GMD and has 12 interceptors on it.
But for _THIS_ system, Aegis Ashore, MIRVing interceptors would be for redundancy (the plan is to launch 2 inteceptors per warhead already, to increase odds of success... so mirving can give many options). However the types of Missiles from Russia they would intercept - Medium and Intermediate Range ballistic missiles - are generally too small to be MIRV'd and carry only a single warhead. MIRVing is reserved for ICBMs and SLBMs.
Furthermore, although it doesnt cover tactical systems, the NewSTART treaty places only somewhat a lower limit on Launchers (800), than it does Warheads (1550), for all bombers, ICBMs and SLBMs. This has generally prioritized swapping to one-warhead-per-missile in order to hit the sweet spot of keeping the entire nuclear structure economical (rockets, let alone warheads are expensive to own while still distributing the arsenal wide enough.
In the US for example, today for the most part only Trident II D5 SLBMs are nearly fully MIRV'd while Minuteman-III ICBMs, while they can be MIRV'd, are only carrying one warhead. This puts the US at 1481 warheads on 741 launchers. Russia, by contrast, puts 1735 warheads on 521 launchers. For the US, this means that Russia MIRVs more. For Russia, it means they have strike 741 sites, 220 more than the US has to strike of Russia. But the US is also richer and can afford more launchers.
In any event, MIRVing is a fraction of what it was 20 years ago, when the latest strategic weapons were built to carry, and did carry, as many as 12 warheads.
Because ICBMs would be mirv'd, not medium ranged ballistic missiles, or Russia's illegal Intermediate Ranged Ballistic missiles, the US can protect itself with mirving GMD in the continenetal US in coming years. European defense will need more SM-3 missiles (probably about 1000+ more, again, well within the realm of possibility IF desired and paid for), but they wont need to be mirv'd to deal with Russia's tactical systems. MIRVing would make them economical (and to be clear, using "MIRV" in this concept isn't the correct usage of the word at all, but you get what I'm saying)
Sorry for spelling. On shit laptop. Traveling for mothers day.