I am not sure that I can provide sample project yet (due to IP) and I am not sure that it would necessarily be helpful because you probably wouldn't be able to reproduce the error. I setup 3 more computers to run DSE with the exact same version Quartus and exact same source. These other three computers didn't have any problems running 3 simultaneously. One of those has only 32GB of RAM and two less CPUs. Thus, it is probable that you wouldn't see the failure because it is not due to the source specifically.
Each time it fails, the stack trace below is given in the DSE window. Does this give any clues as to what is happening?
==============================================================================
Error: Exploration Point a_1 reported:
*** Fatal Error: Access Violation at 0X00007FFD47EFA2C4
Module: quartus_map.exe
Stack Trace:
0x1a2c3: HDB_INSTANCE_NAME::keep_obsv_name + 0x63 (DB_HDB)
0x15095: HDB_SYN_INSTANCE_NAME::keep_obsv_name + 0xd (DB_HDB)
0x41360: HDB_NAME_MGR::keep_obsv_name + 0x2c0 (DB_HDB)
0x3857e: SGN_UTILITY::garbage_collect_names_before_synthesis + 0x2ee (synth_sgn)
0xc72ec: SGN_EXTRACTOR::synthesis_and_post_processing + 0xbc (synth_sgn)
0x13493: sgn_qic_helper + 0x173 (synth_sgn)
0x44d3: qsyn_execute_sgn + 0x263 (quartus_map)
0x14246: QSYN_FRAMEWORK::execute_core + 0x136 (quartus_map)
0x13b9f: QSYN_FRAMEWORK::execute + 0x30f (quartus_map)
0x112a9: qexe_do_grunt + 0xb9 (comp_qexe)
0x16546: qexe_run + 0x356 (comp_qexe)
0x17371: qexe_standard_main + 0xc1 (comp_qexe)
0x1b42b: qsyn_main + 0x53b (quartus_map)
0x133a8: msg_main_thread + 0x18 (CCL_MSG)
0x14bae: msg_thread_wrapper + 0x6e (CCL_MSG)
0x16af0: mem_thread_wrapper + 0x70 (ccl_mem)
0x12c41: msg_exe_main + 0xa1 (CCL_MSG)
0x2a236: __tmainCRTStartup + 0x10e (quartus_map)
0x17033: BaseThreadInitThunk + 0x13 (KERNEL32)
0x526a0: RtlUserThreadStart + 0x20 (ntdll)
End-trace