The Audio that Wonga66 links to, contends, that Rashi says we should have just a theoretical knowledge of how to make the 3rd Temple, but we don't have to actually make it.
I believe he is wrong for several reasons.
1] Rashi in Yechezkel 43:11 says we should learn the measurements so that we know how to do them at the end time, namely, at the time of the building. If Knowledge was just enough he wouldn't have to add at the end time, because Knowledge is always a good thing and not dependant on the end time.
2] When Rashi uses the Hebrew words לעשות or לעשותם the term implies action and not just Knowledge.
So for example, in Dvarim / Deut. 18:9 When G-d tells us "When you come into the land that Hashem your L-rd gives you, don't learn to do like the abominations of those Gentiles." Rashi comments, you shall not learn to do (in Hebrew לעשות) but you shall learn in order to understand and instruct, that is to say to understand how corrupt their actions are and to instruct your sons, do not do such, for it is the manner of the Gentiles. See also Vayikra 26:3 where Rashi seems to be using the word לעשותם to denote actual performance in contrast to just theoretical knowledge of the Mitzva / Commandment which is described by another word.
3 The Talmud quoting Bar Kappara in Tractate Ketuvot page 5, says that the work of the Tzaddikim (righteous individuals) is greater than the creation of the Heaven and the Earth.
The source used by Bar Kappara to prove this, is that a verse in Yishayahu/Isaiah 48:13 describing G-d's creation of the Heaven and the Earth, attributes the creation of each one of them to G-d's hand (in the singular); while in the book of Shmot/Exodus 15:17 it says, "The Mikdash of G-d, your hands (in the plural) have established".
Rashi comments there:The Mikdash is the action of the hands of the Tzaddikim.
4]Finally, According to the Vilna Gaon as recorded by his discipe in Kol Hator chapter 5, the ultimate rectification of the sin of the spies, Meraglim, will be by the building of the Mikdash. This implies to me, physical action by the righteous, because only by this are we demonstrating as opposed to the Meraglim, that our obedience to G-d to conquer and settle the land of Israel outweighs and overrides our fear of the Gentiles. We would not get the same spiritual rectification by just "Learning About the Temple" and making G-d do all the hard work.