Is the Tripartite Pact a full alliance, or is it just a defensive alliance? In other words, are Italy, Germany and Japan all in the same faction? If they are in the same faction, Japan will not attack you as long as Japan remains in the faction. Even if it gets war goals on your territory it won't use them as long as you are allied. I played fascist Czechoslovakia, joined the Axis and refused Sudetenland. Germany never attacked, even with the war goal. You may see the US get war goals on democratic UK if the game runs long enough, because the AI tries to do all of its NFs, and there is one that gives the US war goals against the UK. But if both countries are in the Allies, the US will never attack the UK.
If the alliance is just a set of mutual guarantees of independence, then I'm not sure that they won't eventually declare on you. I advise garrisoning your ports and building infrastructure there just in case...
