A compound of Formula or a pharmaceutically acceptable salt thereof, wherein: R 1 is selected from the group consisting of H, -O-CH 2 -O-P OR a OR b, -O-C -C 1-6 alkylene-L 2 -X 1, O-P OR a OR b, and -O-C -A 1 -(C 1-3 alkylene) n -C 4-7 heterocyclyl wherein the C 4-7 heterocyclyl is optionally substituted with 1 to 6 R c groups; A 1 is selected from the group consisting of C 6-10 aryl, C 3-10 cycloalkyl, C 5-10 heteroaryl and C 5-10 heterocyclyl, each of which is optionally substituted with 1 to 5 R x which can be the same or different; n=0 or 1; L 2 is independently selected from the group consisting of a bond, -O-C -C 1-6 alkylene-, and -NR d -C -C 1-6 alkylene-; X 1 is independently selected from the group consisting of -NR e R f, -P OR a OR b, -O-P OR a OR b, and -CO 2 H; R 2 is selected from the group consisting of H, -L 3 -C 1-6 alkylene-L 4 -X 2, -L 3 - (C 1-6 alkylene) m -A 2 -X 2, -P OR a OC -C 1-6 alkyl, -P OR a NR g R h and -P OR a OR b; L 3 is independently selected from the group consisting of -C -O-, and -C -; L 4 is independently selected from the group consisting of a bond, -O-C -C 2-6 alkenylene-, -O-C -C 1-6 alkylene-, and -NR d -C -C 1-6 alkylene- wherein the C 1-6 alkylene in -NR d -C -C 1-6 alkylene- and -O-C -C 1-6 alkylene- is optionally substituted with NR e R f; X 2 is independently selected from the group consisting of -NR k R 1, -P OR a OR b, -O-P OR a OR b, and -CO 2 H; m= 0 or 1; A 2 is selected from the group consisting of C 6-10 aryl, C 3-10 cycloalkyl, C 5-10 heteroaryl and C 5-10 heterocyclyl, each of which is optionally substituted with 1 to 5 R x which can be the same or different; R 3 is H or -L 5 -P OR a OR b wherein L 5 is independently selected from the group consisting of a bond and -CH 2 -O-; each R x is independently selected from the group consisting of halogen, C 1-6 alkyl, C 1-6 haloalkyl, C 1-6 heteroalkyl, CN, NR y R z, SR y and OR y; each R c is independently selected from the group consisting of halogen, C 1-6 alkyl, C 1-6 haloalkyl, C 1-6 heteroalkyl, CN, NR y R z, SR y and OR y; each R a, R b, R d, R e, R f, R g, R k, R l, R y and R z is independently selected from the group consisting of H and C 1-6 alkyl; each R h is independently selected from the group consisting of H and C 1-6 alkyl wherein the C 1-6 alkyl is optionally substituted with 1 to 5 substituents independently selected from CO 2 H, NR i R j, C 6-10 aryl, C 3-10 cycloalkyl, C 5-10 heteroaryl and C 5-10 heterocyclyl, wherein each R i and R j is independently H or C 1-6 alkyl; wherein two of R 1, R 2 and R 3 are H, and one of R 1, R 2 and R 3 is other than H. R 1 is selected from the group consisting of H, -O-CH 2 -O-P OR a OR b, -O-C -C 1-6 alkylene-L 2 -X 1, O-P OR a OR b, and -O-C -A 1 -(C 1-3 alkylene) n -C 4-7 heterocyclyl wherein the C 4-7 heterocyclyl is optionally substituted with 1 to 6 R c groups; A 1 is selected from the group consisting of C 6-10 aryl, C 3-10 cycloalkyl, C 5-10 heteroaryl and C 5-10 heterocyclyl, each of which is optionally substituted with 1 to 5 R x which can be the same or different; n=0 or 1; L 2 is independently selected from the group consisting of a bond, -O-C -C 1-6 alkylene-, and -NR d -C -C 1-6 alkylene-; X 1 is independently selected from the group consisting of -NR e R f, -P OR a OR b, -O-P OR a OR b, and -CO 2 H; R 2 is selected from the group consisting of H, -L 3 -C 1-6 alkylene-L 4 -X 2, -L 3 - (C 1-6 alkylene) m -A 2 -X 2, -P OR a OC -C 1-6 alkyl, -P OR a NR g R h and -P OR a OR b; L 3 is independently selected from the group consisting of -C -O-, and -C -; L 4 is independently selected from the group consisting of a bond, -O-C -C 2-6 alkenylene-, -O-C -C 1-6 alkylene-, and -NR d -C -C 1-6 alkylene- wherein the C 1-6 alkylene in -NR d -C -C 1-6 alkylene- and -O-C -C 1-6 alkylene- is optionally substituted with NR e R f; X 2 is independently selected from the group consisting of -NR k R 1, -P OR a OR b, -O-P OR a OR b, and -CO 2 H; m= 0 or 1; A 2 is selected from the group consisting of C 6-10 aryl, C 3-10 cycloalkyl, C 5-10 heteroaryl and C 5-10 heterocyclyl, each of which is optionally substituted with 1 to 5 R x which can be the same or different; R 3 is H or -L 5 -P OR a OR b wherein L 5 is independently selected from the group consisting of a bond and -CH 2 -O-; each R x is independently selected from the group consisting of halogen, C 1-6 alkyl, C 1-6 haloalkyl, C 1-6 heteroalkyl, CN, NR y R z, SR y and OR y; each R c is independently selected from the group consisting of halogen, C 1-6 alkyl, C 1-6 haloalkyl, C 1-6 heteroalkyl, CN, NR y R z, SR y and OR y; each R a, R b, R d, R e, R f, R g, R k, R l, R y and R z is independently selected from the group consisting of H and C 1-6 alkyl; each R h is independently selected from the group consisting of H and C 1-6 alkyl wherein the C 1-6 alkyl is optionally substituted with 1 to 5 substituents independently selected from CO 2 H, NR i R j, C 6-10 aryl, C 3-10 cycloalkyl, C 5-10 heteroaryl and C 5-10 heterocyclyl, wherein each R i and R j is independently H or C 1-6 alkyl;