Is this the sort of thing you are trying to do?
library(dplyr)
#>
#> Attaching package: 'dplyr'
#> The following objects are masked from 'package:stats':
#>
#> filter, lag
#> The following objects are masked from 'package:base':
#>
#> intersect, setdiff, setequal, union
DF <- data.frame(Quarter=c("Q1","Q2","Q3"),
V1=c(123,213,333),V2=c(495,372,528),V3=c(45,72,44))
DF
#> Quarter V1 V2 V3
#> 1 Q1 123 495 45
#> 2 Q2 213 372 72
#> 3 Q3 333 528 44
#Sum across rows
DF <- DF %>% rowwise() %>% mutate(Total=sum(c_across(V1:V3)))
DF
#> # A tibble: 3 x 5
#> # Rowwise:
#> Quarter V1 V2 V3 Total
#> <chr> <dbl> <dbl> <dbl> <dbl>
#> 1 Q1 123 495 45 663
#> 2 Q2 213 372 72 657
#> 3 Q3 333 528 44 905
Created on 2020-12-06 by the reprex package (v0.3.0)