Historically, it never happened in recent history that France, England, and Germany cooperated. However, the diplomacy board is not the real world (fortunately).
Generally, a western triple alliance is quite a fair alliance, where each of the powers gain 2 s.c. in 1901, and each has its own targets to acquire in 1902. In 1901 England gains Belgium and Norway, France gains Portugal and Spain, and Germany gains Holland and Denmark, and perhaps even Warsaw as Germany is required to open A (Ber) – Sil.
However, this article is not intended to describe the setting in 1901, rather the situation after 1902. If the western triple is executed (near) perfectly, by 1903 Russia and Italy are hanging in the ropes. And maybe Germany has already crossed the stalemate line into Tyrolia or Galicia. However, it is at this point that the real diplomacy should come to live again.
Western triple alliances usually do not end well. Especially England has little to win after 3-4 years from a western triple alliance. Below is the picture in winter 1902 (before units have been build) when a E/F/G alliance has been executed well. Please ignore the units of the other countries; they have probably moved more than indicated in below map.
Germany is now at 7, France at 6, and England at 5 supply centers. Meaning that combined they already have 18 supply centers. I have played the western triple minimum 5 times, and my personal experience is that the builds in 1902 can already show the long term intentions of the countries involved.
Basically the northern triangle diplomacy has to start at the end of 1902, and the most likely outcome is that two of the countries partner up to attack number three. England has a distinct advantage, it has minimum three fleets, and can build a fourth, while Germany is very low on sea power. French fleets are far away in the Mediterranean. This is also why I believe that a Western Triple is more advantageous for England than France or Germany.
France – Germany
If France and Germany partner up, that decision has to be made prior to the 1902 builds. It is absolutely necessary that they build as many fleets as possible and France has to build one in Brest. This would give them the best chance of moving against England. The key is to get an army on the British islands, the easiest way of doing this is by occupying the channel or NTH, which is not easy at all. The risk of this cooperation is that you lose focus on the fronts in the south-east and countries in this region unite and start a counter-attack.
England – France
If England and France partner up, they can wipe Germany of the board in the normal way. France should demand Belgium from the British, which in turn can take Holland, Denmark, Sweden. Munich is definitely for the French, and Kiel and Berlin have to be discussed. In my opinion, this is the option that is easiest to execute, as most units are already in the right place.
England – Germany
England and Germany partnering up is also a good combination. However, Germany will remain at risk for the rest of the game, as he cannot build many fleets in this alliance. On the other hand, walking into France via Burgundy should be simple, as France has mainly built fleets so far. Also, building an army in Munich will not raise suspicions with the French player.
Continuing the western triple
Is this possible? Yes. Is this likely? No. Triple alliances usually exist at the end of the game. They either exist to get at 3-way victory, or to fight against a fourth country trying to get a solo victory. I believe the western triple fails often after 2-3 years because of the lack of progress that England can make. If England is promised a clear way forward with his armies through Moscow and other parts of Russia, you might be able to keep him quiet and happy within the alliance. Do not underestimate the physiological effect here as Germany and France will usually capture a few more supply centers. This will give the English player the feeling he is not getting his fair share.