I have a data set comprised of 3 conditions - a, b, and c. I want to compare DEG between group a and average of (group b + c), also just DEG between group a in b. In Limma, I would construct a design matrix like the following

design = cbind(a=c(1,1,1,0,0,0,0,0,0),
               b=c(0,0,0,1,1,1,0,0,0),
               c=c(0,0,0,0,0,0,1,1,1))
cont.matrix = makeContrasts(a-(b+c)/2, a-b, levels=design)

I'm trying to replicate my analysis with DESeq2. Here is some test data, and code to show I can extract DEG results between group a and b, b.

However, How do I get DEG between group a and average of (group b + c) ?

test_data=sapply(rep(20,9), function(x) { sample(seq(1:100), x) })
col_data = data.frame(condition=c("a","a","a","b","b","b","c","c","c"))
dds = DESeqDataSetFromMatrix(countData = test_data,
                       colData = col_data,
                       design = ~ condition)
dds <- DESeq(dds)
resultsNames(dds)
results(dds, contrast=c("condition","a","b"))

Take a look at numeric contrasts described here ?results