Well, if somebody does not want WWIII what s/he should do is
1. Immediately stop all negotiations with the Russians on all matters not having to do with the withdrawal from Ukraine. No contacts whatsoever on any matters - however minor or innocuous. The non-essential personnel at the embassy (and families) should be removed ASAP and in a very demonstrative fashion, US citizens advised to avoid traveling to Russia (and those resident there, advised to leave). Consulates outside of Moscow should be closed. Restrict issuance of US visas to the holders of the Russian diplomatic and service passports to the Consular Sections of the US embassies in Kiev and Tbilisi (continue stamping the visas into the regular passports in Moscow, to the extent that the reduced consular representation makes it possible). Russians should be forced to sharply reduce their embassy staff and close their consulates as well. Encourage the allies to take similar diplomatic measures.
2. Immediately impose painful economic sanctions on Russian elites. The Magnitsky list should be expanded to include a few thousand people, at least (including the families of all the Russian MPs who voted to authorize the use of force in Ukraine). Put under the sanction regime all major Russian public and semi-public companies (including the likes of Gazprom). All financial transactions between US and Russia should be put under scrutiny. Russia should be put under notice that unless all troops are removed ASAP, all Russian assets in the US will be confiscated and used to finance the Ukrainian government (any attempt to preemptively remove such assets at this point should be summarily blocked). Impose a special tariff on all Russian exports and imports to be dedicated to financing of the Ukrainian government. Restrict the use of the US airspace to all Russian air traffic.
3. Urgently remove all the restrictions on oil exports from the US. Guarantee, as much as possible, to the Europeans that their energy needs will be taken care of no matter what happens, as long as they join with the sanctions regime. Especially encourage the Turks to join in (they should not be too hard a sell - they are worried). Stress, that Turkish government will get the full support of the US in suspending the action of the Montreux convention on the Straights navigation: Bosporus has to be closed off ASAP. Lithuanians and Poles should be given guarantees of full support in closing the land transit to Kaliningrad.
4. Resume, very publically, the negotiations with the European allies on missile protection systems. Make it clear that this is done in response to the new situation.
5. Start, as soon as practicable, major war exercises with the NATO partners in the region - both around the Black Sea and in the Baltics (as practicably close to St. Petersburg as possible). Reinforce NATO garrisons in Poland, the Baltics and in Norway. Negotiate with the Ukrainian government a few bases inside Ukraine (in friendly, pro-Western areas - but this would not only be Lviv, but also Sumy and Chernihiv, right on the Russian border).
6. Another major war exercise should be conducted with the Japanese, around the Northern coast of Hokkaido. Support should be hinted to the Japanese claim on the Northern Territories. If Georgians and Turks agree, have the Turkish troops take positions on the LOCs near South Ossetia and Abkhazia. Have Moldovans and Romanians start very public negotiations about establishing Romanian bases near Transnistria.
7. At the same time, US diplomats should be rushed to Central Asia and other ex-Soviet states, offering guarantees of protection against the Russian invasion. Those visits should be very public - give them as much media coverage as possible, have Kerry himself go to Astana. The Kyrgyzstan airbase should be reinforced, if the Kyrgyz government allows (make it very hard for them not to allow).
If all this is done in rapid succession and decisively, there is still some chance of avoiding WWIII.
However, I don't think this crisis will evolve into WWII.