At the last count, this dictionary gave expansions for 761 three-letter acronyms, a mere 4.3%, of the 26^3 = 17576 possible. Here's the grep command used:

        egrep '^[A-Z][A-Z][A-Z]$' Dictionary | sort -u

Here they are:

AAP, ABC, ABI, ABP, ACA, ACE, ACK, ACL, ACM, ACP, ADL, ADM, ADS, ADT, AED, AEP, AES, AFJ, AFK, AFS, AGL, AIA, AID, AIX, AKC, AKL, ALC, ALF, ALM, ALP, AMD, AML, AMP, AMS, ANL, ANU, AOS, APA, APC, API, APL, APT, AQL, ARC, ARL, ARM, ARP, ARQ, ART, ASA, ASE, ASF, ASL, ASM, ASN, ASP, ATA, ATK, ATM, ATS, AUI, AUP, AVS, BAD, BAL, BAP, BBC, BBL, BBN, BBS, BCD, BCL, BCS, BDL, BEA, BER, BFI, BGP, BIP, BLT, BMF, BNF, BOF, BOS, BPS, BQS, BRB, BRI, BRS, BSD, BSI, BSL, BSS, BTW, BWQ, CAD, CAE, CAF, CAL, CAP, CAT, CBN, CBV, CCL, CCP, CCR, CCS, CDA, CDC, CDE, CDF, CDL, CDM, CDS, CEN, CFD, CFP, CGI, CGM, CHI, CID, CIF, CIL, CIM, CIS, CIX, CLM, CLP, CLR, CLU, CLX, CMA, CMC, CML, CMS, CMU, CMZ, CNC, CNI, COM, COS, CPI, CPL, CPS, CPU, CRC, CRL, CRT, CSL, CSM, CSO, CSP, CSU, CTC, CTI, CTL, CTY, CUA, CUL, CVS, CWI, DAA, DAG, DAS, DAU, DBA, DBC, DBH, DCA, DCE, DCG, DCL, DCT, DDE, DDL, DDM, DDN, DDT, DEA, DEC, DED, DEK, DES, DEX, DFA, DFC, DFD, DGL, DIN, DIP, DLG, DLL, DLM, DLP, DME, DML, DNF, DNS, DOE, DOF, DOL, DOS, DPB, DPL, DPS, DSA, DSE, DSL, DSM, DSP, DSS, DSU, DTD, DTE, DTP, DTS, DUA, DVI, DXF, EAG, ECL, ECP, EDA, EDI, EDL, EDU, EER, EFF, EFL, EGP, ELF, ELI, ELP, EMA, EML, EMX, ENQ, EOF, EOL, EOU, EPL, EPS, ERA, ERC, ERD, ESA, ESF, ESI, ESL, ESP, ETC, ETM, EXE, FAC, FAD, FAP, FAQ, FCP, FDT, FEA, FEL, FFP, FFT, FGL, FIX, FMQ, FMV, FNC, FOD, FPA, FPU, FQL, FRL, FSF, FSL, FSM, FSP, FTP, FUD, FYA, FYI, GAN, GAP, GAT, GCC, GCL, GCR, GCT, GDB, GEA, GEI, GFR, GHC, GIF, GIN, GIP, GKS, GLB, GLS, GLU, GMD, GNU, GOL, GOM, GPL, GPM, GPV, GPX, GRG, GSL, GSM, GUI, GVL, GWM, HCF, HCI, HCS, HDF, HDL, HDM, HEP, HLL, HOL, HPF, HPL, HQX, HSV, IAB, IAD, IAL, IAM, IAW, IBM, ICE, ICI, IDE, IDL, IEC, IEF, IEN, IFP, IFX, IGC, IGL, IGP, IHS, IIS, IIT, IMP, IMR, IMS, IPA, IPC, IPE, IPL, IPS, IPX, IQL, IRC, IRL, IRQ, ISA, ISE, ISF, ISL, ISO, ISP, ISV, ITS, ITU, IVY, JAZ, JCL, JPL, JRL, JRN, JSA, JTC, JTS, KAP, KBS, KCL, KEE, KFX, KIS, KMS, KRC, KRL, KRS, KSL, LAN, LAP, LAU, LAX, LBE, LBL, LCC, LCF, LCL, LCS, LDB, LDL, LDP, LDT, LEP, LER, LGN, LIS, LLC, LML, LNF, LOC, LOL, LOM, LOP, LPC, LPG, LPI, LPL, LPS, LPT, LSE, LSL, LTR, MAC, MAD, MAL, MAN, MAO, MAP, MAS, MBS, MCA, MCC, MCI, MCL, MCP, MCS, MDL, MFE, MHS, MIB, MIG, MIS, MIT, MIX, MJS, MLL, MML, MNP, MPG, MPL, MPV, MRS, MTS, MTU, MUD, MVC, MVS, NAG, NAK, NAS, NBS, NCD, NCS, NDL, NFA, NFS, NFT, NGL, NIC, NIH, NII, NIL, NIS, NLS, NMI, NOC, NPL, NQS, NSE, NSF, NSS, NTP, OBE, OBJ, OCL, OCR, OCS, ODA, ODC, ODI, ODP, ODT, OEM, OIC, OIL, OLC, OLE, OMA, OMF, OMG, OMT, ONC, OOA, OOD, OOF, OOP, OPS, ORB, OSA, OSE, OSF, OSI, OTI, OTT, OWL, PAD, PAL, PAT, PAW, PBD, PBM, PBX, PCA, PCF, PCI, PCL, PCN, PCS, PDA, PDF, PDL, PDP, PDS, PDU, PEM, PER, PEX, PFE, PFL, PGP, PIC, PIE, PIL, PIP, PIT, PMC, PML, POC, POE, POM, PPL, PPM, PPN, PPP, PRA, PRI, PRL, PSA, PSD, PSI, PSL, PSN, PTI, PTT, PUB, PVM, QAM, QBE, QNX, QPE, RAD, RAL, RAM, RCC, RCL, RCS, RDL, RDS, REC, RFC, RFE, RFT, RGB, RIP, RLF, RLL, RMS, RNF, RPC, RPG, RPI, RPL, RPT, RRS, RSA, RSI, RSL, RSN, RTF, RTI, RTL, RTM, RTT, SAA, SAC, SAL, SAP, SAS, SBM, SCC, SCI, SCL, SCO, SDE, SDF, SDH, SDL, SDM, SDS, SEA, SED, SEE, SEI, SEL, SEM, SEP, SET, SEX, SFL, SGI, SIA, SIG, SIL, SIR, SMG, SMI, SML, SMM, SMP, SNA, SOL, SOM, SOS, SPC, SPG, SPL, SPM, SPS, SPX, SQL, SRI, SRL, SRP, SSA, SSL, STB, STD, SYN, TAC, TAL, TAO, TBK, TCB, TCL, TCM, TCP, TDF, TDR, TEI, TET, TIA, TIP, TLA, TLB, TMG, TNX, TOK, TOP, TPL, TPS, TPU, TRO, TRS, TSO, TSP, TSR, TTL, TXL, UAN, UAW, UBD, UCB, UDP, UFO, UIL, UIS, ULP, UNI, URC, URI, URL, URN, USE, USL, USR, UTC, UTP, VAL, VAR, VAX, VDL, VDM, VEE, VEL, VGA, VHE, VIF, VLB, VME, VML, VMS, VPL, VSF, VSP, VSX, VTS, VUE, WAM, WAN, WCL, WEB, WFL, WRT, WSL, WTF, WTH, WWW, XDL, XDR, XIE, XNF, XNS, XON, XPC, XPG, XPL, XRN, XSB, XSI, XTI, XUI, XVT, XXX, YSM, ZAP, ZIF, ZIL, ZOG, ZUG