Personally I would not suggest mixing more brands of RAM. You already have 2 brands which seem to work together properly. With that in mind I would either suggest 20GB of the Hynix or 12GB of Kingston. But to be sure the most compatible RAM for Dell systems is from Crucial and that is available here.
Good afternoon. I would like to expand the machine's memory set. currently there is 16GB ram in the machine. My goal would be 24Gb ram.3 Kingston 4GB DDR3 1600Mhz KTL-TC316/4G2 pcs SK hynix 2GB DDR3 1600MHz HMT325U6EFR8C-PBN0 I would like to replace 2 SK hynix Rams and add 1 extra ramRight now I can choose from these22 pcs Hynix 4GB DDR3 1600Mhz HMT351U6CFR8C-PB 11-12-B1 (double-sided)32 pcs Hynix 4GB DDR3 1600Mhz HMT351U6EFR8C-PB 11-12-B1 (double-sided)11 pcs Hynix 4GB DDR3 1600Mhz HMT351U6CFR8C-PB 11-11-B1 (double-sided)2 pcs Hynix 4GB DDR3 1600Mhz HMT351U6EFR8C-PB 11-13-B1 (double-sided)16pcs Hynix 4GB DDR3 1600Mhz HMT451U6BFR8C-PB 11-13-A1 (single-sided)7 pcs Hynix 4GB DDR3 1600Mhz HMT451U6AFR8C-PB 11-13-A1 (single sided)5 pcs Hynix 4GB DDR3 1600Mhz HMT451U6AFR8C-PB 11-12-A1 (single-sided)23pcs Samsung 4GB DDR3 1600Mhz M378B5173DBO-CKO 11-12-A1 (single sided)14pcs Samsung 4GB DDR3 1600Mhz M378B5173QBO-CKO 11-12-A1 (single sided)4pcs Samsung 4GB DDR3 1600Mhz M378B5173BHO-CKO 11-11-A1 (single sided)21pcs Samsung 4GB DDR3 1600Mhz M378B5173QHO-CKO 11-13-A1 (single sided)9pcs Samsung 4GB DDR3 1600Mhz M378B5173DBO-CKO 11-13-A1 (single sided)7pcs Samsung 4GB DDR3 1600Mhz M378B5173EBO-CKO 11-13-A1 (single sided)8pcs Samsung 4GB DDR3 1600Mhz M378B5273CHO-CKO 11-10-BO (double-sided)19pcs Samsung 4GB DDR3 1600Mhz M378B5273DHO-CKO 11-11-B1 (double-sided)8pcs Kingston 4GB DDR3 1600Mhz KTH9600CS/4G CL11 (single sided, low profile)19pcs Kingston 4GB DDR3 1600Mhz KCP316NS8/4 CL11 (single sided, low profile)4 Kingston 4GB DDR3 1600Mhz KVR16N11S8H/4 CL11 (single sided, low profile)2 pcs Kingston 4GB DDR3 1600MHz HP655410-150-HYCG 11-11-B1 (double-sided)2 Kingston 4GB DDR3 1600Mhz KTH9600C/4G CL11 (double-sided)4pcs Kingston 4GB DDR3 1600Mhz KTL-TC316/4G (double-sided)(" [URL] ")I can currently order from here. Which Ram could fit next to the current ones?I'm sorry if I didn't use google translate.  
T3500 Ram update