I have the RNA seq data of three tissues of a medicinal plant. My data is without repetition, what package and method should I use for differential expression analysis.

I really need help.

Thanks a lot

For DESeq2, we don't offer any DE analysis for data without replication.

See "What to do if you don't have replicates" in the edgeR User's Guide. I'd suggest edgeR exactTest with a preset dispersion, say dispersion=0.1. It's not rigorous DE but it will at least give a sensible ranking of genes.