I'm a new user of Quantum GIS (1.7.3) and I need to repeat a task multiple times, so I'm wondering if there is a quicker way to do the following (with three or more polygon layers - each layer has a single polygon):

 1. Union
 2. Dissolve
 3. Buffer

The effect will be to have three separate single-polygons shapefiles merged into one (larger) single polygon shapefile, with a buffer around it.

Answers in PostGIS would be useful too as my data is stored there.  Thanks!

**Edit: Adding PostGIS information**

The multiple polygons I want to Union-Dissolve-Buffer are the result of a catchment basin point table that I've created in PostGIS, converted (in R) to an alpha-hull (i.e. concave hull) polygon shapefile using the CRAN alphahull package.  To create the original catchment table in PostGIS, for example, I do this:

    CREATE TABLE catchment_10304 as
    select 
        id,
        geom_4269,
        (select sum(cost) from (
    	   SELECT * FROM shortest_path('
    	   SELECT gid AS id,
    		  start_id::int4 AS source,
    		  end_id::int4 AS target,
    		  newcost::float8 AS cost
    	   FROM network',
    	   10304,
    	   id,
    	   false,
    	   false)) as foo ) as cost
    from node;
    
    CREATE TABLE catchment1_10304 AS 
    SELECT * FROM catchment_10304 WHERE cost < 1364;

Then I do it for three different nodes (representing the boundaries of a region).  To create the total catchment area for the region, I want to Union-dissolve-buffer with the three or more individual catchment area tables.