One of the objectives of this study consists of the assessment of the antitumor activity of several extracts from three selected plant species:Xanthium spinosumL.,Trifolium pratenseL., andCoffea arabicaL. and also a comparative study of this biological activity, with the aim of establishing a superior herbal extract for antitumor benefits. The phytochemical profile of the extracts was established by HPLC-MS analysis. Further, the selected extracts were screened in vitro for their antitumor activity and antioxidant potential on two cancer cell lines: A549—human lung adenocarcinoma and T47D-KBluc—human breast carcinoma and on normal cells. One extract per plant was selected for in vivo assessment of antitumor activity in an Ehrlich ascites mouse model. The extracts presented high content of antitumor compounds such as caffeoylquinic acids in the case ofX. spinosumL. (7.22 µg/mL—xanthatin, 4.611 µg/mL—4-O-caffeoylquinic acid) and green coffee beans (10.008 µg/mL—cafestol, 265.507 µg/mL—4-O-caffeoylquinic acid), as well as isoflavones in the case ofT. pratenseL. (6806.60 ng/mL—ononin, 102.78 µg/mL—biochanin A). Concerning the in vitro results, theX. spinosumL. extracts presented the strongest anticancerous and antioxidant effects. In vivo, ascites cell viability decreased afterT. pratenseL. and green coffee bean extracts administration, whereas the oxidative stress reduction potential was important in tumor samples afterT. pratenseL. Cell viability was also decreased after administration of cyclophosphamide associated withX. spinosumL. andT. pratenseL. extracts, respectively. These results suggested thatT. pratenseL. orX. spinosumL. extracts in combination with chemotherapy can induce lipid peroxidation in tumor cells and decrease the tumor viability especially,T. pratenseL. extract.