The European Space Agency's (Esa) Science Programme Committee (SPC) will meet next month to consider the current status of the candidates.
The committee members will have in front of them a report from an advisory group that has done a ranking on science.
A final decision is unlikely to be made for a year or so.
And to secure the opportunities available, the various competitors will have to show not only that their science is compelling but that their technological ambition is realistic and affordable.
Even then, only two missions can be afforded out of the three.
The concepts include a satellite that would map the "dark Universe" (called Euclid), a probe to study the Sun up-close (Solar Orbiter), and a telescope to find distant planets (Plato).
At the moment, the trio are in the lead, jockeying for position for the earliest launch opportunities in Esa's Cosmic Vision programme. The first launch could come in 2017.
However, the missions still have much to prove if they are ever to be flown by the agency.
Costs division
A further concept known as Spica (a contribution to a Japanese-led infrared telescope that builds on Europe's heritage with its Herschel observatory) is relatively low budget, and again has compelling science.
However, its special nature means it may now be treated in a separate way, depending on how the partner state's side of the project evolves and the budget that ultimately becomes available.
Two other ideas put forward for consideration in the competition have not been put out of the long-term picture. They seem though to be too far from the SPC's budget constraints to progress in this round.
They are Marco Polo (an asteroid sample-return mission) and Cross-Scale (to study space plasmas). Again, these concepts were considered to have excellent science credentials, according to the Space Science Advisory Committee.
But the high prospective cost of implementing these concepts means they would have to acquire additional partners to bring their foreseen budgets back into contention.
Very similar ideas to Marco Polo and Cross-Scale could well return to run in future competitions if the right international cooperation can be found.
Esa has budgeted up to 475 million euros (£430m) (at 2010 prices) for its contribution to the Cosmic Vision winners. The expectation is that the cost of the instruments that fly on the spacecraft will be borne directly by the member-states that provide them.
All the concepts were presented to the scientific community in December in what amounted to a kind of space "beauty contest".
