Attempting to recode elements contained in a list

Hello R-Masters,

I have a list 's' with three elements, as shown below.  I want to recode 
a.a, a.a2, and a.a3 to NA if the value in a.a is less than 3.

s <- data.frame(scan=letters[1:3], time=LETTERS[1:3])
s$a <- array(runif(9)*6, dim=c(3,3), dimnames=list(NULL, c("a", "a2", 

s

I reivewed my Modern Applied Statistic Book, the online help and did 
some searching of R-help on the internet. I explored unlist and 
as.list.data.frame in an attempt to isolate the third element of the 
list s, but this was not helpful. Any help would be appreciated.

dim(s)

[1] 342   3

names(s)

[1] "scan" "time" "a"

s

                 scan               time a.a a.a2 a.a3
202       Plastic lids       Plastic lids NaN  NaN  NaN
195       Plastic lids      Cookware lids  44   31   58
205       Plastic lids  Seasoning packets   9    4   20
191       Plastic lids      Baking sheets  15    7   27
209       Plastic lids           Utensils   9    4   20
194       Plastic lids        Cell phones   0    0    7
193       Plastic lids       Canned goods   0    0    7
197       Plastic lids    Hand/dish towel   3    0   12

Thank you.

Greg Blevins
The Market Solutions Group

Windows xp, 2.1.1

Gregory L. Blevins
Vice President, Partner
The Market Solutions Group, Inc.
gblevins at marketsolutionsgroup.com
Office phone: 612 392-3163
Cell phone: 612 251-0232
"What is at the beginning but a small error, swells to huge proportions at the close." Aristotle