The only thing the two of them together could do is dominate Europe really, and from there the Middle East and perhaps even China. I doubt they could successfully cross the English Channel, and they sure as hell arent going to cross the Atlantic, and and invasion of the US from Alaska isn't likely to succeed.
First of all, I'm going to assume that, notwithstanding political differences to the contrary, ITTL Nazi and Soviet leaderships have enough commont sense to avoid backstabbing each other at least as long as they have a common enemy to fight, otherwise, it defies the whole purpose of the exercise.
Let's assume that either the Soviets join the Axis in late 1939 (either because the Franco-British DoW the Soviets when they invade Polony, or they bomb Baku or intervene in Finland and the Soviets DoW them), or in late 1940, when the German-Soviet negotiations to enlarge the Axis to the USSR are successful.
Let's also assume that the USA join the British in a timeframe running from late 1940 to late 1941, for various reasons (e.g. naval clashes with the Axis in the Atlantic or Japan attacking South East Asia). With the USSR in the Axis, American isolationism shall be significantly weakened.
A Nazi-Soviet-Italian alliance would be the undisputed master of Continental Europe, North Africa, and the Middle East.
It is theoretically possible but extremely unlikely that the Americans could amass and land enough troops in North Africa in time to prevent the British from being overrun and the Axis from entrenching in the area and turning the Mediterranean in its own lake.
I don't think there are any realistic chances that the Allies could avoid the loss of the Middle East to the Soviet onslaught (esp. because neither Turky nor Vichy France would be in a position to refuse opening their borders, ports, and airports, to the transit of Axis troops in Turkey and Syria).
The Anglo-Americans may have better chances to keep India (even if significant chunks of the North shall likely be overrun by the Soviets and Japanese) IF Indian nationalist movement decides to stay loyal to the Allies as it did IOTL.
A Nazi-Soviet combination of forces makes a successful Sealion a realistic outcome, just as it is the A-A successfully defending the British Isles from landing attempts. It shall be a tight race between the Axis and the Allies accumlating enough local air-naval power to allow or defeat the landing.
A successful conventional Overlord is extremely unlikely, as it is a successful Allied landing in mainland Italy, even in the (remote) chance they could ever manage to secure North Africa. The Allies would have at best air and equipment parity and naval superiority vs. manpower inferiority. They could maybe manage to secure sizable portions of western Iberia or Scandinavia, or possibly in Greece/Turkey in the unlikely case of an Allied North Africa, but they could never use them to make substantial inroads in Continental Europe, and with the Axis tapping Soviet natural resources and Middle Eastern oil, the amount of damage the Allies could make from their peripheral footholds (say by bombing Swedish ironfileds or Romanian oilfields) is questionable at best. Any such Allied footholds would most likely become huge Anzios, a drain of resources for both sides.
Most of China would fall to a Soviet-Japanese alliance, even if guerrilla would continue.
However, no matter how much the Axis would hold the advantage in the conventional field, the A-A hold the ultimate ace in the hole in terms of the Project Manhattan. The Axis shall never be able to match them in time here.
Hence, the most realistic outcome is the Axis securing Europe, North Africa, and the Middle East, and temporarily overrunning South East Asia. The Allies achieve a stalemate in India. China falls to the Axis, even if guerrilla continues with smuggled Allied weapons. The Axis fails to land in the British Isles (but they have a decent chance of succes, with some preparation, so it's not a given), and the Allies fail to land in continental Europe (at best they secure some footholds in Portugal and Norway, but they fail to expand them). The Allies slowly reconquer the Pacific and South East Asia. At the end of the 1940s, the Americans have amassed so many nukes that they are able to overwhelm Axis anti-air defenses and nuke the Axis into submission. Unless a) the Axis pulls a Sealion before, or b) they achieve a rudimental MAD by blackmailing Britain with nerve/radiological warhead missiles. In the latter case, it is a strategical stalemate, which ushers in the Cold War.