The intention is that Israel leaves Egypt for the land of Canaan (Exodus 3:17). Now they ask to make (only) a three days’ journey into the wilderness to offer sacrifices to God (Exodus 5:3). They were not lying here, they just started modestly (a feast in the wilderness, as God has told, Exodus 3:12): they can test whether Pharaoh is willing. Later they can ask for more.
At the same time God tests Pharaoh: does he recognize him as superior?
1 Moïse et Aaron se rendirent ensuite auprès de Pharaon, et lui dirent: Ainsi parle l'Eternel, le Dieu d'Israël: Laisse aller mon peuple, pour qu'il célèbre au désert une fête en mon honneur.