Yes, it can be done with LR3.
1 - Choose a photo and then go FILE > EXPORT to open the export Menu.
2 - Scroll down the window to the 'Watermarking' option.
3 - Click the 'Watermark' option box and then select 'Edit Watermark' from the drop down box.
4 - Select the 'Text' radio button in the top right hand corner.
5 - Type the text you want for the watermark in the text box below the photo
6 - Adjust the text size, colour, opacity and position using settings in the right hand panel.
7 - Save the watermark.
8 - Make sure the watermark is selected on the main window.
9 - Click 'Export'
Once you have created your watermark, there is no need to create it again. You can just select it from the drop down menu when required. I've created two different ones and choose which one I require. I select all the photos I want to export, go to the Export options, select the watermark and whatever files format/size and let it do the work.
You can also create a PNG file of your logo/text in Photoshop and use that instead of text. You just click on 'Image' instead of 'Text' in Step 4.
That's what I've done for mine:
Hope that's of some help.