Unearth has two main meanings, a literal one and a figurative one. The literal meaning is “to dig up something that was buried in the earth.” Example: “The archeologists unearthed Qin Shi Huang’s tomb.” The figurative meaning is “to discover some fact that was hidden.” Example: “The journalists’ investigation unearthed the facts about government corruption.” Dredge also has two main meanings, one literal and one figurative. Literally, it means “to dig up the ground under water to make the water deeper.” Example: “The government dredged the bottom of the bay so that larger ships could enter the harbor.” Figuratively, it means “to bring up an old facts that another person would prefer to forget about.” Example: “Every time she fights with her boyfriend, she keeps dredging up old facts about things he did wrong in the past.” Burrow, as a verb, means “for an animal to make a tunnel in the ground to live in.” As a noun, the burrow is the tunnel. Sometimes, burrow gets used figuratively to mean “investigate facts.” Example: “The journalists kept burrowing into the situation, trying to figure out the truth.”

conclusion with above, unearth has to dig out and explore meaning that usually underneath the earth and mud. dredge is 淘 so it is to dig up deeply and move mud become a river or lake. and burrow is to drill a hole to let animals(rabbit) to live. also, unearth and burrow have to discover the facts that people investigate meaning. and the dredge is to 翻出旧帐 figuratively meaning. I learnt this daily words but I hope I can remember it. So I answer you to impress them and let you understand if you are not.