Having a fixed connection is good, and using the Airport Express will give you potential, full CD-resolution.
However, you'll find that you'll have to turn the volume of the signal down quite a lot unless you want very loud volume.
And the disadvantage with that is that you could then be reprocessing the signal inside your MacBook before it goes to your speakers. (Depends upon which version of iTunes you're using.)
The solution is to either use a passive attenuator - though you'll then be feeding your BL3s from the Airport Express DAC (digital-to-analog conversion, and that's not a DAC that will match your speakers.)
Another problem is that there are two different audio processors in a Mac. One is the core audio processor controlled by the utility Audio MIDI Setup; the other is the Mac's SRC.
Read through the following to get a feel for the weaknesses in Apple's sound card structure, and how to ensure you bypass them. (I'm a huge Apple fan, but they could have made this simpler.) The methods are different for iTunes 6.x and iTunes 7.x
For optimal playback quality, matching the abilities of BL3 and BL5, the instructions offered by Benchmark must be followed. Here's a description of the problem:
CoreAudio and iTunes can simultaneously operate at independent sample rates. At all times, the sample rate set in AudioMIDI Setup dictates the sample rate at which CoreAudio is operating. When iTunes is launched, iTunes locks to the sample rate at which CoreAudio is currently operating (which is the sample rate that is set in AudioMIDI) and does not change until it is closed and re-launched. However, after iTunes launches and locks its sample rate, its sample rate will not change thereafter, even if CoreAudio's sample rate setting in AudioMIDI Setup is changed. To change the sample-rate of iTunes, iTunes must be shut down, and then restarted after the desired sample rate is set in AudioMIDI Setup.
http://www.stereophile.com/budgetcomponents/108bench/index5.html
http://www.benchmarkmedia.com/wiki/index.php/ITunes-QuickTime_for_Mac_-_Setup_Guide
The first link outlines the problem, the second specifies the cure.
Your BL3s are so good that they deserve to be connected to a DAC that will feed them a properly converted analog signal, using the optical out from the Airport Express. This DAC would then also let you control volume, without touching the optimal settings on your MacBook.