Not so bad. Assuming a spherical Earth in a vacuum and a bunch of other slightly less reasonable assumptions we have:

1E+45 photons from sun per second

1.49598E+11 distance to sun in meters

6365000 earth radius in meters

2.81229E+23 surface area of solar flux at earth orbit

1.27276E+14 surface area of earth exposed to sun

4.5257E-10 ration earth area to solar flux

4.5257E+35 photons hitting earth per second

6.49E+15 one light year in meters

1.947E+25 meters in 3 billion light years

4.76367E+51 surface are of sphere 3 billion ly across

9.50046E-17 photons per m^2 at 3B ly

1.05258E+16 m^2 per photon

57883255.23 radius in meters for mirror to reflect 1 photon/second

57883.25523 radus in km (Only 10x larger than Earth!)

Then given a reasonably competent engineer to make a descent mirror with no aberration and a good telescope on a clear night away from the city, and a good bit a patience, you could watch a replay of the last 3 billion years.